I need help!!!

So, my boyfriend is really into BDSM. While I don't mind that in the slightest, the topic of us doing something like that together has been coming up a lot recently. The issue with this is I've been trying to recover from PTSD from sexual trauma I experienced in previous years. We've been going at a slow pace as far as anything sexual is concerned, which I really appreciate because he's been super attentive to whether I'm comfortable with something. But I haven't told him about the PTSD or what caused it yet and I don't know how, but I know I have to. He's on a two week trip right now so that gives me time to prepare, but I really have no clue how to go about even bringing the topic up, let alone carrying on the backstory and telling him I don't know what I'm comfortable with yet. But since he has a strong sexual drive I'm afraid it will change the way he looks at me in the relationship. Someone please give me some advice, I'm completely lost here