I'm going to take the bull by the horns--let's say she did think you were a loser and she was "trading up."
So who did she pick? In your words, someone with a track record of messing around. Someone who doesn't believe in commitment. Why commit, when you can have several women at your beck and call?
So she swears fidelity to you for life, but the second she gets bored, she finds a replacement?
This woman sounds like someone who I'd want to think that I'm a loser--a loser who believes in love, marriage, faithfulness, being a father, being a provider--yes, what a loser! Most women would KILL for a man with just some of those qualities.
Guess what--when a loser thinks you're a loser, you're even--wash your hands of them and walk away. Her body, her face, her voice, that's just the outer shiny package that is hiding narcissism, moral corruption, rot. She took a good thing and threw it away for a chance with a man who is no better than she is. She is the loser.