Coriandre's right (love your avatar, btw

). However, paranoid people find it extremely difficult to just ignore the source of their paranoia. They're inclined to consider "turning your back on a monster" as "ignoring a monster and letting it rage on behind your back". It's good that you've informed your boyfriend about your paranoia, sugar. I know you don't want to be fucked over again, but consider that your boyfriend doesn't want that for you either! What kind of person would hurt you after you've so seriously expressed your fears for such.
Why would he cheat on you under the awareness that you've been cheated on before, and it's something that's always bothering you? He puts up with your paranoia for a reason - because he wants to be with you. He sounds like a pretty decent guy, and it's important that you let him know you think such. Stress that the problem is yours, not his, and how you appreciate his empathy.
Consider all the girls out there who have never been cheated. It's happened to you once, and it's not likely to happen to you again. Especially since your boyfriend knows how sensitive you are to it. You've established that you are paranoid, he will always be aware of it, and now it's time to move on. It would be highly foolish of him to cheat on you, given your super-sensitivity, as you would probably find out.
As Coriandre said, focusing on a certain possibility increases the likeliness of that possibility to be made reality. It's important that you concentrate on making your relationship a happy one, embracing the good things and appreciating what you have. If you strengthen your relationship in this way, your boyfriend shouldn't think twice about endangering it.