trust issues

So I've caught my boyfriend talking dirty to other girls in the past, usually on twitter or texting. We've been together for almost 4 years now. I really want to believe he has changed. But whenever I ask to use his phone he gets all antsy. I've noticed he has been deleting a convo on fb with some girl,  from what I read before he deleted it it was just casual talk. He also takes his phone in the bathroom with him. I cant help but to think he's hiding something. I need reassurance that he's not hiding anything, but I'm not sure if he just deleted it bc he didnt want me to get the wrong idea??? He literally brings his phone everywhere with him.  Idk what to do. I'm bad with awkward confrontation and I dont want to get in a fight.