I slept with my ex after she was engaged
Why would you sleep with your ex if you knew she was engaged?
I wanted to break up her relationship.
Sleeping with your ex when you already knew that she was in a relationship just for revenge is wrong. Let go of your past and don't get involved with your ex.
I didn't know that would get into this problem. I just wanted her back.
You should talk to her and let her know it was a mistake. Let her handle how she will tell her fiancee.