ktbug, welcome to the paranoid-defensive-can't-trust-anybody-club!

If you've been together for 2 1/2 years, I suggest that you just tell him as it is. "I went through such and such, now I'm all messed up because of the trauma and I have a hard time trusting people (perhaps, especially men)".
I'm the exact same way, and everybody who experiences something traumatizing - beit abuse, cheating, drug problems whatever - will have a hard time trusting someone again.

It's normal; it's just a defense mechanism. It CAN go away by itself, but that takes a lot of time and frustration for your bf. The easiest (though not really) way to let your defenses down is by confronting your traumas and yourself. I mean REALLY confront them, not just say "I know this and this happened".
For me, it's been one trauma after the other from I was 2 years old till I was 19 - and they all kinda fueled my crazy-fire.

My current bf and I almost broke up because of the way I behaved, and I was forced to pick at my old wounds and find the root of it all.
When I figured it out, I told him everything - and since then, I've had NO problems trusting him. None! Defenses are down, the paranoia is practically gone, and I think I might be trusting him a bit TOO much lol.

Once you tell him everything, and you realize that he's still sitting by your side and isn't gonna go anywhere - it's easier to handle the trust-issues. If he's had to deal with you not trusting him for two years, it's safe to say that he's not gonna go anywhere and that he loves you very much.

But I do suggest that you see a psychologist and open up to him/her about this, you can be vague with your bf about it until you figure it out. In my opinion, I think he will appreciate that you're aware of your issues and that you're working on them. Cos being with someone who doesn't trust you is incredibly hard.

That you're aware of your issues is in itself progress! I didn't know that I had any trust issues until September this year!!! o.o

Best of luck! Feel free to PM me if you have any questions or something. There are plenty of people on this forum who are just like you, you're not alone on this.