I actually think it's good that you've recognised that this is an issue you have and not the relationship. I've seen cases of people so paranoid about cheating they lock their partners up. They actually believe that nasty voice in their head.
It's the same voice I get that tells me I can't do anything and everybody hates me. The first thing I did to help me deal with it was to detach myself from it. It's there, and it's loud, and you end up hearing it, but that doesn't mean you have to listen or believe what it says. That's the first step.
Try then to allow it to be there. Don't fight it, don't resist it. I've been doing this for a while now and the voice is gradually getting less and less potent. What it does signal to me is where I need to do some emotional work. I use the Sedona Method to release emotions and fears and have found it incredibly effective. Alternatively you could CBT to rationalise with yourself. Personally, I've found the Sedona Method more effective.
Hope that helps,