1-First of all, try hard.
When your friend/boyfriend/girlfriend/family/teacher/anyone do something terrible to you, or at least, when you think that was terrible, the first thing you do is probably get mad or get sad, Specially when that person is important for you.
Anyways, now you have two options: forgive the person and try to start over or give up on a full relationship because of one mistake (unless the person have done something that hurts you before, or even a lot of times, and you can't handle anymore, then I think you should forgive, get away a little and try with another person). But, if you still feel like the person worth the effort, GO GET IT! Don't give up, talk to him/her and try to change the things, say what you feel and ask the person what him/her feels, listen to each other's opinion and don't lose your hope on him/her.
2-Forgiving is not forgetting.
As I say in #1, you can forgive and forget THE PERSON. I mean, don't forget AT ALL, but change your relationship with her, get a little far. When you forgive the person, don't forget the mistake. Watch out, because you never know when someone can make a mistake again. Be prepared to anything around you.
3-Listen to your parents/closer friends.
I'm saying this because someone close to me always says that "the person into the situation never realises what is going on with it", it means that you never see the whole situation as it really is if you are a member of it. You see a part of it. Who really sees almost 100% of it (and are more experienced than you) are your parents, or a close friend. So, listen to them and what they have to say about the person - if you should forgive and forget him/her or try again. But you must put your thoughts on it too, make your point together.
4-Have courage to apologize.
Just because you made a mistake, it doesn't mean you don't deserve forgiveness.
5-Calm down.
When you feel like you are almost going out of your mind of pure madness, take a walk. Because you will end up saying and doing things you were not supposed to say and do in your normal spirit's state. It doesn't mean you can't yell, have arguments and say the truth, it means that when you are saying wrong things, you should calm down.
6-Please, don't hate.
Ok, the person has messed up. That's because you are dealing with an human. You can make mistakes too. What I mean is: no one deserves to be hated.
I hope I have helped you, guys ;) always make wise choices, ok?
XOXO