I hope that conversation with your friends goes well! And I'm glad they've been willing to believe you when you tell them what happened; from the sound of it, the fact they're outside of your family and therefore not bought into the narrative of your cousin being good is a big help. I completely understand that frustration with the lack of physical evidence; it often feels like, if you just had a concrete piece of proof, everyone would believe you. But it's worth keeping in mind that, even if you did have physical proof, some of your family members might react the same way they are now because they're so invested in seeing your cousin a certain way. So while it's totally okay to feel that frustration, it can also help to channel some of that "if only" energy into taking care of yourself.
Do you want to brainstorm how to raise the topics of shame and self-hate with your therapist, so that when you're ready it's not as daunting to do so?
I also wonder, do you think it's worth talking with your therapist about feeling physically vulnerable and talking about options other than self-defense class for helping you feel more protected? Although there are an increasing number of self-defense classes designed for disabled folks, so you could also check those out. This article lists a few, as well as how to find ones in your area: http://aabr.org/2019/06/26/self-defense ... abilities/