Relationship

My boyfriend & I have a really happy and healthy relationship, and we don’t fight that much (when we do it’s small dumb stuff) but his best best best friend who he’s really close too and has been with years, is now dating his ex. He use to be really serious with his ex. I’m really insecure (due to anxiety) and worry a lot. My boyfriend isn’t around her, but I just don’t think it’s okay at all. My boyfriend is loyal and I have no reason not to trust him, I actually trust him just not the friend or ex gf. I know my boyfriend would never do anything to harm or hurt our relationship, and if they’d stay together there is NO way I’d want him around her, or I’d go around her myself. It starts fights when I bring it up. All my friends say I’m just being dumb and I shouldn’t care, but I do... and I don’t know what to do?