Hey Kate,
Me and my boyfriend were together for two years and he cheated on me - a one night stand which he told me about 3 days later.it was a really rough time and I kicked him out so that I could have enough space to think things over. After a month of grovelling I decided to give him another chance (because in all honesty I missed him) - before he moved back though, we decided to lay down some rules - one of which was that he wouldn't go out on the piss until i trusted him 100% which he didn't.
Like I said, it was a really hard time but once he earned my trust again by doing what I asked of him I had to draw a line under it. It was tempting to bring it up in arguments (and sometimes something can set me off in a mood) but we're happier than ever now. We are now a lot more honest about our feelings and this helps SO much.
If don#t trust him yet, sit down and explain to him that if you want to be together, you're going to have to set the rules for a bit and if he wants to be with you, he'll do it (with a timelimit set)
Hope that helps xx