That can be hard to accept because it means that you've been used to this idea of an objective reality outside of yourself all this time. A reality which has its own inherent qualities and which reports them to you as though they are distinct from your perception.
What it implies now is that what you're experiencing as reality, is in fact a hallucination that's part of your own mind's malfunctioning, that you literally are inventing everything that bothers you, that you are doing all that to yourself.
This implies also that there is no fixed reality out in the world, and therefore there is no inherent truth based on form. You literally give everything the meaning that it has for you.
ACIM lesson #2, right at the start: "I have given everything I see…all the meaning that it has for me."
This means that you are basically inventing every single justification that you think is truth, or that you think is based on objective facts outside of you. It means that, if you want to, and when it suits you, you MAKE UP everything that you think is true.
If you are in favor of something, you will make up reasons why it is a good idea, to support it and lend weight to your conviction that it is somehow inherently right. And if you are against something, you will make up reasons why it is a bad idea, in order to continue to support your belief that the reality outside of you is fixed and has nothing to do with your experience of it.
It can be perhaps alarming to discover that you are literally making everything up as you go along, and that all the things you thought were factual or true are really just your rationalizations. You decide what stuff means. You lend it credibility and pretend that this credibility comes from outside of you.
You think that other people's opinions, authority, certifications, education, experience, or even physical evidence, are all ways in which the world has an inherent truth. You have come to depend on this notion that you are not experiencing the results of your own made up subjective fantasy, but that everything that's going on is happening "on its own" without your input. And because you do not see the ways that you contribute to it or indeed create it, you do not accept that IT HAS NO TRUTH.
This leaves you in a kind of limbo like state where you're kind of just hanging there with no sense of what the truth outside you is anymore. If you're making up all the meaning, and quite literally the entirety of humanity is totally making up the meaning of everything in society as it goes along, then really there IS no inherent meaning outside of your mind.
This means the world is basically either neutral or insane, and that, based on the ego, it CANNOT make sense. There cannot be truth to it, and since it is a mental hallucination, it is nothing better than a chaotic illusion. Trying to "understand it" is an attempt to discover that it has an inherent meaning of its own, separate from your mind. We are very addicted and conditioned to continually look for meaning outside of ourselves, in the world, in order to disown true meaning (God/Self).
This is what happens when you try to figure things out, diagnose sickness, analyze situations, look for what is really happening, try to find proof, explain the background of something, try to identify causes, look for rational explanations or depend on the opinions of other people's authority. This is you attempting to turn an illusion into a reality, trying to convince yourself that you are NOT the one who has given it all the meaning it has, or that it is not just some blank canvas onto which you've projected an entirely fantastical world.
What this means for your mind and your sense of reality is that you cannot continue to claim that there is an objective world out there, and nor does that world have its own meaning at all. It is in fact a meaningless world, and this meaningless world cannot be understood unless you transcend it.
ACIM lessons 10 through 14:
"10 My thoughts do not mean anything."
"11 My meaningless thoughts are showing me a meaningless world."
"12 I am upset because I see a meaningless world.
"13 A meaningless world engenders fear."
"14 God did not create a meaningless world."
If the world is meaningless, and God did not create it (thus it is not objective reality), then this shows you that every meaning you see in it is made up but your thoughts, which are themselves meaningless and contain "pretend meanings". These pretend meanings attempt to give meaning to a world which has none.
What you may be afraid to admit is that it is not possible to find truth in any way shape or form in the world, because the world is not real. And that means your mind, your sense of reality, has been believing that it's had nothing to do with what you perceive all this time. When in fact it is the sole inventor of all the meanings that you think the world has.
Everything supposedly "out there" is neutral and therefore if you think something is good or bad or right or wrong or acceptable or unacceptable it is ONLY because you're making up a truth which you try to turn into reality and try to justify to yourself as being objective. It is not. It is subjective hallucination. So if there's no truth out there, it leaves you sort of hanging as to how to deal with reality.
It is as though you create your own reality, which is a popular new-age concept, which also leads on to manipulating the world. But this really is denying the fact that you do not create reality. You are creating UNREALITY. But it's a step closer to realizing that what you are experiencing is coming from you, and therefore what you perceive is not necessarily objectively real.
You're dreaming, and only God creates reality!
This is why in your awakening journey you then have to go through the process of recognizing the part that your mind plays in inventing meaning, how it distorts perception, how you contribute to what shows up outside of you, how you use it, how completely subjective you're being, and how everything that you thought was objectively real is in fact not real at all.
As you do this you will gradually realize more and more that there IS an objective reality, it's just not "out there" as a physical world, and instead it is within, in your Oneness with God. Formless, timeless, spaceless, ageless. The ONE and only meaning. A meaning that has not been fragmented into a billion conflicts. A meaning which hasn't been artificially attributed to an illusion. A meaning which ACTUALLY IS MEANINGFUL to you - that you can feel in your heart, in your very being.
God is true meaning. God is the one and only meaning. All else is meaningless.