I'm going to make my ex-girlfriend cry.
That's cruel! Why would you make her cry?
She was fooling around with another guy. How can I not have some kind of reaction to that?
If she was cheating on you, I understand being angry at her, but intentionally being mean to her won't help things. It will just make things worse and make her more angry at you. If she is simply seeing a new guy after you already broke up, there's nothing wrong with that and you need to move on.
Yeah but I want to make her feel bad about what she did.
It's understandable to want to get revenge on someone who wronged you, but it's best to just move on and forget about her. You will be happier if you focus on making your life better instead of trying to get back at her. Try to find a way to spend your time that makes you happy!