My boyfriends best friend.

So I'm 17 I've been dating my boyfriend for about 6 months and it seems to be going good I guess. He's very serious about us and our future together, but I can't seem to get on board with him. Before we started dating his best friend (who is also my best friend) and I weren't technically in an official relationship but we were really close and flirty with each other. We both liked each other but for some reason it never really got going. I had really strong feelings for him. When me and my boyfriend got together my best friend and I were on a bit of an outs since my now boyfriend told him to back off. But now months later my best friend is showing interest again and old feelings a resurfacing on my behalf. I love my boyfriend but I've always sort of been in love with my best friend. I don't know what to do. My boyfriends really serious about commitment (which scares me) and I'm really attracted to my bestfriend. HEELLPP!! Any advice would be helpful