I don't trust boyfriend what should I do?

I been with him for 4 years he's my first real relationship. I try trusting him but I just can't he has never done me wrong. He's honest with me. I'm the one that has cheated on him allot of times he knows I have so I kinda feel like He mit of done something like for a pay back u know. I catch him looking at girls And I tell him what the **** and He tell me what Ur tripping i wasn't even looking at no one I don't even know about what girl ur talking about. I really don't know what to do Some days I can't even sleep just thinking about him cheating on me or doing things Biden my back I'm real jealous I try being that girlfriend that u can tell everything to like what girls he thinks are hot Some days I don't mind I don't trip about it but other days I'm like wow He's stupid I don't know what to do PLEASE Help!!!