I cheated once and I won't tell
Why have you decided not to tell?
It makes me feel like an awful person.
It is bad to cheat on others. It might be something you should disclose. Your partner has a right to know.
Nah, I don't want to hurt her feelings. She would be angry.
It is understandable that she would be angry if she found out you cheated. Cheating is a betrayal and she deserves to know the truth. She might break up with you but she might also forgive you.