Is my boyfriend in love with his best friend?

My boyfriend used to have with this girl right after he got a divorce, they were nothing serious just messing around, no big deal right, well hes known this girl for years, they basically grew up with each other and I found out that he had actually ended up falling in love with her and she ran scared because shes scared to commit, she is a very very promiscuous and cannot be a faithful woman. It had actually broke his heart. Later on they decided to just be friends with benefits which they were until i came around. He quit sleeping with her for me which is amazing for me but she still comes around and hangs out and im not the kind of woman to keep a good friend from anyone but, every time she is around its almost like he forgets im there and he treats me like a friend not a his lover. He does in ways of course but he pays more attention to her than me, they talk continuously and sometimes they seem to flirt not touching but verbally and when she needs to leave he gets upset and in a flirty way tells her she cant leave yet that he wants her to stay longer... She is currently having with his best friend who has recently moved in with us, which is also fine but he is not the same with me like he used to be and since she is having with our roommate it seems like it just gives her an excuse to come over... I know for a fact they are not having or he is not cheating on me but Is he still in love with her. should i just let him go? because at times i feel like it would be best to just let them be, but I know they wouldn't get together because she's too into drugs and she couldn't be faithful to him but sometimes i feel like i'm only good enough because the best to him can't be. what should I do please Im hurt and in desperate need of advice