My bf having feeling for another

Okay here’s the story... so my boyfriend had feelings for one of my best friends long before him and I dated and they sorta had a thing for like literally a day and then she broke it off with him for another guy, then a couple of months after him and I got together at a gathering, it was so unexpected but we both caught feelings and I told him I wanted to become friends before we dated so we did just that and then after like a month or two of being just friends we decided to make it official. Now a couple of months down the line i get these weird thoughts in my head that he still likes her, he’s always mentioning her, he talks to her probably more than I do and she’s one of my closest friends. Idk how to feel? I’ve tried talking to him about it but it’s just too weird. What do I do?