Please help. We argue all the time now?

I'm a very jealous person and I tend to over think everything. It's starting to cause a lot of arguments between my girlfriend and I. I want to be able to trust her more but I don't know where to start. Not that i have any reason not to trust her I just have trust issues. She was my best friend before we started dating but I constantly think I'm wasting her time or that she could do better than me because I get so jealous. It makes me act like a douche bag sometimes but I don't mean to be. I care for her a lot and I don't want to lose her over me being jealous when I have no reason to be.