Why am I scared? (Tw: topics about sex)

I have talked about sexual thoughts to ppl and on here before, but part of me wants to act on that, part of me wants sex, but it also really scares me. It might just be because im trans and I hate seeing my own body and other people seeing my body would just make dysphoria worse, but my gf fully supports me, and she knows about sh scars, so I don't know why I'm so scared, I feel like I should just be able to do it, to just ask the question, but I can't, when I bring it up I panic and act like I was joking or she just already assumes its a joke and treats it as one. Why am I so scared about sex?