I slept with my ex's mom.
What made you want to sleep with your ex's mom?
She's cute and I find her attractive.
It's was not a good idea to sleep with your ex's mom no matter how you feel about her. Doing so could cause a lot of tension between your ex and their mom, and nothing good will come of it. You should break of the relationship right away.
I'm not telling him. Let him find out. I want him to get really angry.
Are you doing this just to punish your ex? That's not the right thing to do, better to just move on from the relationship.