Hahaha how does it feel to know that your "best friend" is happy to bang your girlfriend. Stop being a bitch, grow some balls and move on. If you let people step all over you, you're gonna have a bad time.
Drop that bitch like a bad havit, cause if you don't that's what she will become lol for real though. Forget the year you spent with her, better to have a good year and done than a good year followed by another shitty one or until you hate her more than you already do.
First of all, she's a cheater and it WILL happen again.
And as far as wasting a year of your life and money on her, look up Sunk Cost Fallacy.
Basically, she doesn't love or respect you and you should leave her.
I dated a girl for 4 years and broke up with her after she cheated on me. I then married my wife of 8 years 6 months later. Sometimes, you just gotta let them go son. It also wasn't a waste. You learned a lot in that time. What you want and don't want and don't need. Specifically that -blam!-.