How do trust him again we been together for a year and we only cheated once but it hard

This won't be the answer you want, but you can't trust him again because he's not trustworthy and you know it. It's time to move on or you will be dealing with more than the same. Even if he's always perfectly behaved from here on out, you will never feel secure that he isn't cheating, or that he doesn't want to cheat.

Fortunately, he's your boyfriend and not a husband. Be mindful of the purpose of dating - it is to find your husband. The reason we start with dating is to have the opportunity to see if we are compatible and if the other person is a good match for us, and if they are the kind of people we want to be bound to for life.

So, the dating process worked for you as it should, and helped you identify that this person is not deserving of your trust. You might say, "but I love him and I don't want to break up". Well, that's useful, too - not to stay with him though. Consider what you love about him, why you are reluctant to break up. Whatever those qualities are - his good looks, his sense of humor or whatever it might be - are things you can determine are important to you, that you should seek in your next mate.

Many men and women have tried to change people to turn them into the right mate, and none have succeeded in the history of time. The smart people have looked for partners who are a good fit to begin with. You won't find anyone who's perfect but you will find someone who's imperfections are mere annoyances to you, not serious breaches of your trust. Find the guy who annoys you by leaving his dirty socks in the living room, but only has eyes for you and treats you well as a matter of course.