I'm sorry that you're having so much trouble with it... even if you're worried about it you can't let it control you life to where you can't go out with your best friend to lunch.

The fact is that if he's going to be unfaithful you can't stop him by watching him like a hawk. And him seeing that you don't trust him will not help the relationship at all because trust is important (I know it's hard - I'm not being critical or anything but you and I both know that it's important in any relationship to have mutual trust).

Do you have a reason not to trust him, like has he betrayed and/or cheated on you in the past? I think you really need to make an effort to let it go enough that you can regain your social life - isolating yourself from your friends will just make things worse overall because you are pushing away your best support system.

Are you seeing a therapist or have you tried anti-depressants? They can both be really helpful in my experience.