Unloyal boyfriend and how to deal?

So I just found out that my ex boyfriend (we had a mutual break up about 2 months ago bc we go to different schools and school is really hectic) had been cheating on me the entire time... I had a lot of respect for the guy. And now I feel so stupid. The girl he'd been cheating on me with (and is currently dating) was his good friend and classmate. I met her once and she was sorta my friend. Now I know that all of his study sessions with her had little to do with actual studying. He was my first boyfriend and we went through a lot together. I don't want to be petty, but that's all my brain can think of. Just a million ways to get back at him. What should I do? How should I be feeling? I don't have any romantic feelings toward him anymore, I'm just really hurt by this and don't know how to function.
Unloyal boyfriend and how to deal?
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