You have gotten some great advice and by people qualified for giving such advice.
I’d second, third, forth or eigth the therapy option.
However, I know from experience about this co-dependency, though I am not sure my therapist called it this, as I just had a bad boyfriend that I had reason to believe he was cheating. He was a cheater, he’s always been a cheater, and to this day he continues to be one. However, I am not in the relationship, but I still suffer from insecurities I gained in that relationship.
But here I am about to get married to a wonderful, trustworthy and spiritual man. I am not saying you need to get religious, but faith has helped me. My FH was in the seminary and we talk about the insecurity. If you aren’t to a place in your relationship where you can voice your concern then best that you get outside help. If you are at a place where you can voice your concern, be warned that he might think you are accusing him of something. This should not be the case. stay away from "you" statements, these emotions belong to you, therefore they must be "I" statements.
I am not saying you are a cheater, but in my experience (past boyfriend) was all freaked out about my every whereabouts, and it turns out that those that cheat are more likely to suspect others of cheating. I don’t mean for it to seem as bad as that might have just sounded.
There is a reason you fear someone cheating. Maybe one of your parents did, or a friends parents, or you perhaps in a pervious relationship, or you know that in a previous relationship your partner did.
But I caution you, don’t go playing private investigator. It will ruin the relationship and waste endless hours or days of your time.
Try to relax. When those thoughts come to your mind, push them out, change what you are doing. Try and think about something wonderful he did for you, he is after all dating YOU. And that is a choice. He clearly finds you good enough to have around his friends or family or to be seen with you in public. Enjoy the time you have with him. When he’s not around occupy yourself with something you really love, and if there isn’t anything, find something. Or else you will drive yourself nuts.