To borrow a phrase, you're just not that into her. If you were, even with a few drinks on board, you wouldn't have cheated.
You seem to be blaming everything/everyone but yourself: the alcohol, the chica, no doubt the weather patterns. YOU DID THIS. If this is the way you're presenting it to her, you guys are done already.
My guess is that she dressed and followed you because she has had doubts for a while. I guess you confirmed them. You wanting to leave her at home and go to a party without her is a BIG red flag. I don't care if she has a broken nail and doesn't want to go. You call your cousin with your regrets, order a pizza, and go hit Redbox. Yes, even if she says "No, you go..."