I feel terrible for doing something that I should not have done. It is killing me inside.
This sounds serious. What did you do to her?
I cheated on her.
You should say sorry when you do something like that. That's terrible to cheat on a loved one.
Yeah yeah I'm going to write a letter and make myself feel better.
It might take more than just a letter because cheating is one of the worst things you can do to your partner. What you need to do is care about her, do stuff for her, and somehow earn her trust back, which is if you are lucky. Remember, you messed up, so it is about her now and not how you feel.