How do I get over this boy?

I mean your three options are probably 1. Stop talking to him, or 2. Tell him how you feel (this works for me every time), or 3. Continue on the path with nothing changing. If you tell him how you feel (which I advocate for most of the time), you'll be able to get it off your chest. And if you guys are strong enough friends and he doesn't like you back (which is weird because flirting thing???), you guys will get over it. I had to tell my best friend that I wanted to marry him basically, and he said that he didn't like me like that. I had one bad night but afterwards our friendship had never been better.But I agree with Wulffang, if he's just tryna get into your pants. Nah girl, nah.

Well just think about it, if he has been with this girl for 3 years, which is a good chunk of time, but when she isn't around he acts sleazy such as groping you wouldn't you be upset if he was your boyfriend? You probably just find him to be attractive so you just have to find a means to move on. No one can really tell you how to do it you know? Whether it's by not going to the parties so you don't have to see him or just doing things you enjoy doing. But you should definitely stop him from the groping that's pretty inappropriate. I wish you all the best, have a good one.

Don't talk to him anymore, because he obviously doesn't respect both his girlfriend and you by flirting with you. If he's been with her for three years and still flirts with you, then it only means he likes getting attention out of you but would never pursue anything more than a flirtationship. The best option would be to move on and seek a relationship with someone who will respect you.