It’s not surprising to me that you are having trouble sorting out the truth. You are under enormous external pressure. Your mom’s suicide attempt in the wake of your disclosure scared you. Your family thinks you are a liar. Your father keeps telling you you “ruined” the family. You want to be with your mother. Meanwhile, you are also struggling with the internal pressure of either coming to terms with a trauma or a mental illness. It makes sense to me that you keep moving back and forth between believing it happened and thinking it didn’t. You are responding to those conflicting pressures.
You’re now in a bind. If it did happen, you have to come to terms with having been molested by someone who was supposed to love and protect you. If it didn’t, you have to deal with whatever motivated you to lie. Neither “truth” is comfortable, so you stay stuck.
I can’t help you determine the truth on the basis of a letter. The most important thing you told me is that you aren’t talking with the person who can. Counselors who specialize in PTSD are usually well trained in how to help someone who is struggling as you are. I have a guess that you aren’t doing your therapeutic work because you are afraid of what you’ll find out. That may be just the place to start with your therapist.
Your therapist can’t help you out of the bind unless you get invested in doing your therapy. Please give your therapist a chance. You really don’t have to go through life suffering like this.
I wish you well.