You have to as yourself a very important question. Do you worry because of you, or because of him? What I mean is, we can carry past hurt and pain from relationship to relationship. Did you have a guy who used to talk to others in a previous relationship? Are you projecting? If you can honestly say no, then it’s how this current guy is making you feel. He, for whatever reason, doesn’t seem to be trustworthy. If that’s the case, then you have to be honest with yourself about him.
I have talked to him about my overthinking and insecurities and he says he wont because theres no reason to cheat on someone you love and i agreed but iam i guess stuck in my mind about his past relationship and that for some time i saw him looking at her yet he denies it and gave me alot of reassurance