I'm the exact same way, though I have paranoid PD. My bf can say and do sweet things, but as soon as he talks about another woman - I forget all of the sweet things, and I think he's cheating.

It's REALLY hard to get over those paranoid thoughts, it's a constant struggle. I find myself slipping back into paranoia cos of trivial things, and I have to slap myself out of it and force myself to focus on the nice things. I write down the nice things he say to me and the things he do, and it adds up - so when I feel paranoid I just read it and it calms me down somewhat.

I compare the paranoid thoughts to flies buzzing all around me, whispering lies in my ears. You need a flyswatter.

My advice/battleplan:
1. Learn how to notice when a paranoid thought starts telling you lies
2. Don't argue with the paranoid thoughts, don't start saying "but this" and "but that" - just stick your fingers in your ears and go "lalalalala I can't hear you, lalalalala". Not your actual ears, but you know what I mean.
3. Don't let the paranoia take over your thoughts; don't let it go so far that you can't stop thinking about it. Keep your mind occupied on other things.
4. This is what was incredibly hard for me; find out WHY you're paranoid. For me, it was because of traumas and too many heartaches, and I was trying to sabotage for myself. Before you know the reason behind it (everything has a reason behind it, disorder or not), you can't begin fixing it or at least trying to control it. I had to pick on old wounds and it was really painful, and I had to map my life and scratch my brain for two weeks (alone on a soulsearching journey) before I was able to say "Oh...". It was like I was laying a huge jigsaw puzzle, and I had all the pieces - but I was missing that one piece that connected them all. Then I found it, and then it made sense.
5. Granted that step 4 was successful; tell your man about what you discovered.
6. Every time you notice your paranoia, just tell yourself that you know why you're thinking that way - and you'll notice the paranoia is much easier to deal with.

According to your profile, you have schizoaffective disorder, and sadly paranoia and delusions are a part of that. But I have delusions (delusional jealousy) and paranoia as well, and I try my damnedest to keep it controlled. If you can't figure out why you're paranoid, it's a lot harder. I still get paranoid thoughts, but I stick my fingers in my ears and try to ignore them.

It's hard as helm, but the fight is worth it. Also, talk to your bf about it and tell him about things that he does that triggers the paranoia - and ask him to stop doing it. I did that with my bf (took a couple times before he got it), and it's soooooo much easier now. And he tries his best to include all details about his whereabouts so that it won't give me a reason to worry; cos he knows that I won't tell him and that I'll just end up feeding the paranoia if he doesn't give the details.