You’ve done everything you can to assume your responsibility for your part in the incident with your friend’s boyfriend. But you are going way overboard. No one made him kiss you or touch you. He bears as much responsibility as you do. Your friend’s issues at this point are with him, not with you. I suspect she knows this and that’s why she didn’t trust him. So, yes. Leave that part alone.
Of more concern to me is how much you are beating yourself up for a momentary lapse in judgment two years ago. You are doing what is called “ruminating.” You are going over and over a bad thing without ever letting it go. Ruminating is often linked to depression.
There are coping techniques that sometimes help. For example: Whenever this incident comes up in your mind, you can distract yourself by thinking about something positive. You can meditate, pray, or do something that requires concentration. The idea is to break the cycle of ruminating by shifting your thinking to something else.
If that works – great! If not, you may want to consider seeing a therapist for an evaluation for depression. Indicators in your letter that you are depressed are your statements that you don’t deserve to feel better and that your character is flawed.
If depression is indeed the problem, your therapist will recommend treatment that may include some medication as well as some talk therapy. With appropriate help, you can let go of the past and get on with life.
I wish you well.