I'm in love with my best friend who is also my ex but he has a gf
Have you confessed this to your friend?
He doesn't talk to me anymore.
Dating someone who is already in a relationship is a bad idea, especially because there are so many people involved and in the end someone always gets hurt, often in more ways than one. Try to talk to someone about your feelings and see if you can move onto someone else.
I just want to hurt him back and make him suffer.
Feeling rejected and hurt is okay, but not okay to hurt him. He is in a relationship and you can't expect him to drop her. Try and date someone else for now.