I think my boyfriend has a thing with his bestfriend

This guy I'm talking to is going to prom tonight with his "bestfriend." She's a beautiful girl but I've never met her. Sadly me and him aren't exclusive because he said he wants to focus on Graduating and I understand. He didn't ask me to prom because we are on and off and he already asked his bestfriend before me and him began talking again. I'm a sophomore and so is his bestfriend. When I left school he said he would call me later and never did. After 5 hours I checked his Snapchat and his "bestfriend" left a video of them cuddled up on the bed listening to a song by Jacques that says "When we f****** I be catching chills..." it hurt me so bad but he's not my boyfriend so what can I say. Now he's calling me and tonight is his prom but I'm just over it and us. I've been put second through our entire time together and when we see each other he says childish things and then kisses on me. He's actually becoming annoying so should I just b like f him and us?