Tell him that you have a problem with trusting people but that you were not always this way. (Don't give him the specifics about past romances because that could become a problem in itself. It will be a turnoff.) Ask him for his advice on what can be done to make you feel better, more comfortable, and back to your old self again. . Men like to solve problems. He will be flattered that you asked for his opinion. And you have served him notice that you have issues in this area. Hopefully when you say or do something in response to your fear, he will cut you some slack by not taking it too personal as if you did not trust him. Good luck!
You can't blame him with what past boyfriends have done to you. You will have to trust him until he gives you a reason not too. Don't drive him away with jealousy and constantly harassing him about what he's doing when your not together.
Get counseling. The man you're with NOW doesn't deserve to be measured by the jerks from your past!! The issue is in your head, not by anything your current BF has done. Do him & yourself a favor & seek professional counseling.