Hi Diana
I’m no great source of wisdom, but I hope these two thoughts are of some assistance.
When I was much younger, I had a boyfriend who was still friends with his ex girlfriend. I really struggled with it because I knew he had really cared for her and had been very distressed when she had ended their relationship. I felt jealousy, and I hated feeling that way. But gradually just with the passage of time I came to realise that he wouldn’t be with me if he didn’t want to be, and whatever he had had with his ex girlfriend, it was different from what he had with me, and him spending time with her had nothing to do with our relationship. And over time they spent less and less time together, and our relationship grew. I just had to learn to trust him and trust in our relationship. Looking back, I think if I had continued to focus on his relationship with her, and that he was still spending time with her, all it would have done was damage our own relationship, which went on to become one of long standing. So trust in your boyfriend, and if you don’t, perhaps try and look at why.
My second thought is just this – it sounds like they had a fairly casual sexual relationship, and that neither of them wanted anything more. People regularly feel attracted to someone else in a casual way that has nothing to do with love or deeper feelings or connections. It seems to me that he has found that deeper connection with you, and no longer has a need for the casual relationship he had with her. That’s not to say he doesn’t still like her as a friend, and I think it is quite natural for you to have had some insecurity over this. But it seems to me you need to focus on what you and he have with each other right now, and turn your thoughts away from the past. It is his past, you also have a past, but what happened in his past is not a reflection of what he has now with you.
My only other advice is that perhaps seeing a little less of her, and not getting together with her, will minimise her place in his life and her importance in yours. Get out there with him and with other people, and let her gently and slowly fade from being such a feature in your present relationship.
All the best.