Well, there are two things here. You feel like you're lying. So go ahead and lie and write the thing, for the sake of clarity. Then analyze it.

The other part, meldown, is because of self-judging. Judging too much is harmful. Separate that off, think about it, read about it, analyze it, whatever it takes to get clarity on that.

The one thing is poisoning the other thing that, as you say, you know you are perfectly capable of doing, writing the letter. Sometimes being analytical can help separate off harmful mental behaviours, as in this case, maybe.