Being unforgiving really means that you are looking at the appearance of illusions and are believing what you see. This means that we don't recognize that we're being faced with an illusion, and instead think that it is somehow the truth. We've fallen for the trickery, for how it 'appears', but are not overlooking appearances to see the truth.
This is the same thing as not seeing Christ in others, because you can only see Christ in others when you are overlooking the illusion. i.e. you have to see past appearances, and that means also seeing past the body, seeing past how things seem, and being very clear about the fact that people are actually Christ.
If they are Christ, they are immortal, powerful, have freedom of choice, cannot be victims, and cannot die. They are also the ones dreaming their dream. If we lose sight of that we fall for the illusion they're creating and take it to be real. Doing so is 'judgement'. We are 'judging' what we see, evaluating it, engaging with it, instead of overlooking it.
It doesn't just mean we are judging harshly, labelling people, pointing out flaws or condemning. Even if we believe for a moment that someone dying of cancer is 'real' and that the person isn't choosing it, that is judgement.
So what is a failure to forgive? How does judgement justify that failure? What is forgiveness? Forgiveness is a state of mind in which you are so forgiving that you see only innocence, truth, Christ, and do not for a second fall for any illusions. You know the truth, you know who is behind all this, and you can no longer believe the trickery - appearances.
When you fail to do that, you are attempting to explain to yourself why you stopped being forgiving. So you have to point out that it was 'because of' things that you are not responsible for. It was because of other people. I mean look at them?! And so we become tangled in appearances, forgetting to overlook appearances, because we are trying to make out that we were right to not forgive, and that the person actually 'is' or 'deserves' what you're seeing in them.
What we need to realize is that this entire world is an illusion, which means, someone somewhere is deliberately creating something for the sole purpose of pretending to be something else, for portraying that author as 'not the author', to try to convince him/herself that it's actually true, and to make-believe that this story is really occurring. It's nothing more than like when a child plays make-believe and pretends that stuff is happening.
This story is what is being shown to us all around us all the time, including our body. And it's our challenge to see through the make-believe, not take it seriously, know that it is not true or real, and be aware of who it is that is making all this seem to happen. It's only because we do not recognize the author of it (and therefore do not recognize ourselves) that we think it is real. And not recognizing Christ as Christ (as the dreamer of all this) means to fail to overlook the illusion and instead fall for it, which means we are judging it to be the truth - that it is truly awful, truly deadly, truly upsetting.
It's not true. The dream is just a dream. We need to wake up to realize who is dreaming all this.