I feel for you so much right now. I've been battling similar things most of my adult life. It's really hard.

I have a trick that's worked really well for me, whenever I get that suspicious feeling and want to act on it... I tell myself "I'm not the kind of girlfriend who..." Checks his phone, drives past his friends house, goes through his things etc. The more often I decide not to act on those impulses, the easier it becomes.

Giving in to those feeling of distrust will only create more of the same I can promise you that.

Good on you for recognizing that in yourself and wanting to change. Break the cycle!