Women don't decide "Well, I really like this guy better than my boyfriend, BUT, I'm a good girl who turns down better guys". If he was better than you, then you need work, not a woman with part of her brain removed.
If a woman you love leaves you, she is doing you a favor by inspiring your improve or else suffer. Some men here think that once they get laid the woman should be required by law to never have sex with another man again. Don't we wish.