Overprotective mother

Okay so I'm 22 and tomorrow night my bff and I are taking a road trip to the city to visit a college friend and go to the bar. 
I am in a stable relationship with the same guy for going on 3 years. He's fine with me going without him. 
My mom is so mad that I'm going without him. She keeps thinking I'm going to get drunk and cheat. It's very insulting. 
She also doesn't want me staying at my friends house because she thinks he's a bad guy which he isn't. She has only met him once breifly. 
I'm so frustrated with her. I feel smothered. 
What should I do?!