To accept the Atonement is to accept that God's truth is the truth. It also means accepting that nothing else is true.
Jesus demonstrates this when He speaks of His own state of Atonement:
"There is nothing about me that you cannot attain. I have nothing that does not come from God. The difference between us now is that I have nothing else. This leaves me in a state which is only potential in you."
We would be perfectly willing to accept God's truth, IF we did not believe that something else is true. We have been listening to another "voice", the voice of the ego (untruth) rather than the Voice of God (truth).
If we believe the ego has some truth to it, it means we agree with it. We will defend it, protect it, want to keep it, and will shield it from God. We'll use it as justification for "why" we believe we cannot accept God's truth as true. This literally makes us perceive that God's truth is NOT true.
This is the meaning of "He who would not forgive MUST judge to justify his failure to forgive." We are busy justifying WHY we cannot accept atonement/forgiveness, in order to keep and protect the ego that we believe is sacred and true and worthy of us.
No-one in their right mind would want to accept something they believe is not the truth. No-one would want anything to do with something they regard as "false". The ego attributes all qualities of falseness to God, including perceiving Him as evil, as a threat, as an attacker, and as willing that we be dead. That's quite a lot of false belief to "reverse" - thought reversal - if we are to become genuinely WILLING to want to be with God.
An unwillingness to accept God's truth comes solely from a willingness to accept the ego's truth. Another way the Course puts this is in terms of faith. We have "faith" in the ego, we have come to TRUST it, we think it is helping us, protecting us, defending us, acting in our best interests, and is "working" for us. We think we WANT what it is offering. This is because we do not RECOGNIZE the ways that the ego is deceptive, a lie, a form of attack, self defeating, suicidal and fixated on death.
When we believe the ego is the truth, we literally become "blind" to the truth, and incapable of discerning that God's truth is true. We see falseness IN it, because there is falseness in our perception. We see reflected in God our own "evil" content, thinking it is his.
For example, whenever we misperceive the truth through the ego, we see it as attacking us, we put up a defense, which implements blocks to life, which produces sickness and death. This production of sickness is an attempt to "save our life" by protecting us from life. This is obviously insane. The ego's efforts to "help" us is actually an attempt to deceive us, to trick us into hurting ourselves "in the name of" helping ourselves. In other words, the ego's advice produces the opposite result from what it promises.
While we believe the ego has truth to it, we will not be WILLING to accept the atonement. In fact we'll not be able. We can't ACCEPT it, to really deeply abide in it and feel it and welcome it and want it and admit its truth and ALLOW IT to love us. That's because we have allegiance to the ego and "want" what it is offering instead. AND YOU CAN'T HAVE BOTH! You have to choose. This valuing of the ego's offering produces DOUBT. Being in two minds, or even entirely in ego, will make us very uncertain about whether the truth even IS true, and whether we even DO want it.
Failure to discern what IS true, and what IS false, forms the basis of this confusion, which makes us unwilling and unable to accept it. No-one wants to accept what they think will destroy them. It's just that, believing the ego we will believe GOD is the one who is going to destroy us, and the ego is our savior. It simply "makes sense" to us not to accept God's version of events, because His truth does not "make sense" when the ego perceives it.
If we believe there is really death, really punishment, really fear, really victimhood, really guilt, really sin, really separation, really sickness, really suffering, we will not be WILLING to accept the atonement. The atonement says the exact opposite is true. This means we WILL have to become willing to RECOGNIZE all of these illusions for what they ARE, how we are using them to support the ego, and how they are not TRUE or real.
So we are going to have to systematically let go of all belief that there is such a thing as real sickness, real death, real sin, real guilt, real fear, real suffering, real victims, etc. To do this, we will have to be a LITTLE WILLING to believe that the ego beliefs do NOT contain truth, they in fact are FALSE, and something else is MORE TRUE. If any of this ego stuff - illusions appearing to be real - is within our perception, i.e. within our attitude or beliefs about ourselves, it has to GO, BEFORE we will genuinely, deeply, feel WILLING and READY to accept the truth of God.
Each time that we are attempting to perform a "forgiveness", i.e. we're upset and suffering in some way and we want to get back to the truth, to apply the Course principles and heal ourselves, it's going to have to entail the UNDOING OF FALSE BELIEFS. i.e. the undoing of listening to the ego's interpretation, stopping listening to its voice, stopping believing its bullshit is true, discerning HOW and WHY it is not true, and RECOGNIZING the illusions - i.e. the lies - that you are perpetuating. This can take tremendous radical HONESTY. Honesty admits the truth - allows the truth to enter.
Applying forgiveness SHOULD also therefore mean, that you start with wherever you're at, with whatever you are experiencing now, and you need to transition from there ALL THE WAY TO THE ATONEMENT. If you do not go all the way, if you stop short and keep some beliefs, you will not "complete" the forgiveness because you're not getting to the point where you FULLY ACCEPT the truth and FULLY LET GO of the ego.
While over time you generally will move gradually towards truth, on the whole, you actually can take yourself ALL the way to the atonement at any time, on a SPECIFIC topic. For example if you have issues with a parent or a coworker or something, some belief that is getting triggered... to FULLY forgive that issue, or at least a big chunk of it, you need to look at the beliefs involved and undo them, ALL the way until you get to "sin did not happen in reality, there is no sin at all, therefore I'm still innocent".
Taking back a projection, for example, is only a FIRST STEP in forgiving. It only means then that you recognize some measure of responsibility, that it's actually "about you" and not "about" another person. That's a vital step, and taking of responsibility is essential to any application of forgiveness. But that's not the end of the story.
The problem is, even if you can recognize it's not about other people, you may still believe you are afraid, you are guilty, and you are sinful. These beliefs have to be uncovered as well. Part of the problem is, there are various stages of separation: sin > guilt > fear > punishment > death and each stage entails a shitload of DENIAL. That denial makes content in your mind UNCONSCIOUS. That means you don't even REALIZE that you have this stuff in your mind somewhere, buried, behind a wall of illusions.
So if all you do is take back a projection and then sit with it and don't really realize how you have unconscious guilt lurking in your mind, which your state of victimhood was made to cover up and distract away from, you may not even get to the stage of becoming AWARE of that guilt, to undo it. And even if you get to where you are undoing the guilt, you may not even be aware - there is such a things as UNCONSCIOUS SIN. That has to be undone as well. Only when all of these layers of DENIAL in the mind are undone, ONLY then will you find yourself actually WILLING and able and ready to wholeheartedly ACCEPT that the Atonement is true. That is the point where forgiveness is COMPLETE.
You can do this on any given topic. It doesn't have to just be the overall whole issue of separation. Every time you forgive all the way to the atonement you receive a miracle, and then can offer it. Every time you make a genuine effort to becoming WILLING to accept the truth - your innocence, that literally nothing happened in reality - your mind gets healed one more giant leap towards God. As much as you can accept at the time. This will have PROFOUND healing effects on your perception.
There was a woman that I couldn't stand, a social worker, who had poor sense of boundaries and was always overstepping her role. I had fear about seeing her, anger about her behavior and various other upsets. I thought she was the problem. Later I became willing to see how I was actually seeing some unhealed stuff about ANOTHER person, in this person. I applied forgiveness through various steps and went all the way to atonement on this issue about that person. This was a strong healing.
Later, when I saw this woman again, my entire perception of her was so different. She seemed like a completely different person, as though I'd never seen her before. I had no reaction to her and saw no "sin" in her. She didn't seem to bother me and I knew that I'd been accusing her of something that I thought someone else was guilty of, but which *I* was guilty of. The interesting thing is, when you lift off of other people your own sin, it doesn't necessarily make that person disappear. Instead, you see them more clearly for what is there, and what is theirs. It's interesting then how their OWN call for love suddenly becomes obvious, distinct from anything going on in yourself.
Anyway. I have many other examples where when I've gone all the way to really thoroughly undoing false beliefs and accepting God's truth, it's healed my mind in a major way and really cleared up my perception. It has allowed me to see "truth" where once I saw a distortion. Or rather, to see correctly and accurately where I once only saw what I wanted to see.
If you have been forgiving in some way but not going ALL the way to atonement, I recommend giving it a try. Even if it's on just one small topic. Forgiveness is only complete when you recognize there literally has not been a "real sin" in reality, that whatever it was only took place in a dream, and therefore has had no real effects.
Atonement DOES cure everything. There was an accident that happened years ago where I hurt someone. It produced tremendous guilt. A lot of things happened because of it including a major depression, ostracision from society and various other nightmares. I healed a lot of it over the years and rationalized in various ways why it was ok or not ok or whatever. But I could never QUITE really let go of it. That is, until I went to the atonement with it.
Going to the atonement on this, I FINALLY got to the point of realizing that what I thought happened DID NOT HAPPEN in reality. It only happened as part of a dream. All of my other healing efforts had been to somehow justify "what happened" or to explain "why" it was ok. That never healed me fully. I had to undo the belief it happened AT ALL. When I finally could accept and SEE and RECOGNIZE that it is TRUE, and makes sense, that I REALLY DID NOT DO ANYTHING, and nothing real can be threatened, I could actually willingly ACCEPT that God's truth is true. I could ACCEPT how God sees me. I could ACCEPT that I must still be innocent. And then there were tears of joy at FINALLY being able to LET GO of this "sin" and RECEIVE GOD's LOVE!
Only atonement can be said to cure. Miracles express from the atonement.