What is wrong with me..

I have serious trust issues because of past relationships, everytime I get into a relationship I get cheated on. I have been in this relationship for 6 months and that's the longest I've been in a relationship (I'm 17) I know he'd never cheat on me but I get so insecure when he hangs out with his friends it's unreal, like I just want to be able to trust him cause he's never given me a reason not to.. I just don't know how to fix these issues...