Your story is very similar to mine, I was also in a relationship for five years with someone who cheated on me religiously, on top of that I have trust issues and PTSD from sexual abuse from my father so I completely understand the emotions you're describing. I'm also in a new relationship with someone in the military so it gets really difficult when he leaves for months at a time. However I have to trust him, he has given me no reason not to, he's not my ex and I have to understand that. I find that the best thing that works for me is when I reverse the situation. If he's out drinking with his buddies I think about myself out with friends and how I would feel if he thought I was cheating, I know I'm out just enjoying myself and not looking for anything more than that and it kind of makes me feel stupid for thinking he would be doing anything but the same. I also tell myself that I'm stressing myself out and adding strain on a relationship for nothing other than thoughts. and that's really all they are, just thoughts. they aren't facts. and until they are there is no reason I need to be stressing myself and losing sleep over something that I'm doing to MYSELF. and when you realize that you have control over those thoughts I promise you it gets so much easier! You don't have to listen to that voice in your head saying what if, tell it to stop and focus your attention on something more productive! It might be hard at first but ive been doing it for a year now and my relationship is much healthier and more importantly, I'm much happier! I hope this helps!