I'm going to tell my now ex gf that I had sex with another girl
Why do you want to tell your ex this?
I want to get back at her because she cheated on me.
I understand your need for payback.  She cheated on you, and that's a terrible thing to do.  But don't you think you're being petty about this, since you two aren't together any longer?
She deserves to get hurt like I did.
I believe the best and most proper, positive thing to do is to just live a better life than her. Success is the best revenge, and you will be much happier not clinging on to the negative past.