I can't stop checking my bf's phone

So my bf and I have been together for about 3 years. Since the beginning of our relationship, he's gotten random texts from old hook ups and ex bfs. I would have never known except about 5 times early in our relationship we were cuddling and looking up something and a text will come through. Over the years, I've occasionally checked to make sure no one was texting him. If they were, I saw he didn't respond.When I see him move his phone now or act strangely. I can't help but check to make sure nothing is going on behind my back. I know it's not right and I know I shouldn't have trust issues. He just burned me early in our relationship, and I'm having trust issues. What should I do?