You don't trust him because he's done things that prove he is not to be trusted.
The point is that, there is nothing you can do about that. He made his choices and they had nothing to do with you.
There is no way that you can keep an eye on him 24 hours a day to make sure he doesn't cheat on you again. You have to let that go. He is going to do what he is going to do. All that you can do is find a way to be at peace with that or find a way to get away from it.
L