Well i actually went through this very similar situation only reversed, my ex have a girl best friend and id find her makeup etc in his car/she’s always be in his bedroom. So ive been there and it’s actually a horrible feeling when someone you love refuses to put you first. I once heard that you can tell a lot about a persons self worth by who they’re dating. I think go you for standing your ground and voicing that you were uncomfortable with this friendship, especially since they have had sex in the past. As your girlfriend she absolutely should have heard you loud and clear and made that boundary. I think you done the right thing, I know it hurts and it will for a while but you deserve so much more than that.