Boyfriend troubles

Hi! I am seeking some advice from you fellow ladies. I have talked to one best friend about this but I’d like to hear other opinions that aren’t biased.

I am dating a guy for about 4 months now. We get along great and there has not been any problems with arguing or anything like that. He has a girl best friend that he works with for over 5 years now. They have hooked up twice in the past drunk. They still seem too close and flirty to me and I’ve chosen to let it go or try to these last few months. I feel like everything revolves around her, almost like he is in love with someone he can’t have.(she’s married and has kid)

I don’t know if I’m over thinking it or being too needy. I can’t tell him about his friend because I live in a small town and I’m sure he’d end up discussing it with her. I’m not positive. I don’t know what to do. I’m thinking of just ending it tomorrow but that feels impulsive.

Please help!!