If you want a new start, you'd better start to trust people only up to 80%! Sometimes, it is occasion that makes people cheat on you, don't give the occasion by trusting at 100%! That way, you can forgive when someone cheat on you; but i don't think we should forget, it will help us to know what kind of relationship we can have with each and everyone!
To forgive for real, you must first find a reason; try to understand why that person cheated and imagine yourself; would you resist or did that person respect you as much as you trust him? Then, build the respect, make it mutual!