I'm depressed about kissing a trans person 4 years ago. I feel suicidal; I have chest pain.
Why do you think you feel so strongly about the kiss?
I was a retarded lad back then. Moreover, I hang out largely with gays and the subject has been brought up a lot.
If your feelings are overwhelming you and you need help, you should see a therapist. It is bad to use slurs like 'retarded' or talk in hateful ways about LGBQT people, so maybe speaking to a therapist can help you deal with those issues as well.
I think I can handle this on my own.
I don't think so. If this is still bothering you after all this time, then you need to get professional help.