Why do men always dump me for the “better woman” but even they don't last with the “better woman” they left me for?

Because those men are jerks. And those women weren't better, they just weren't expecting much because they weren't yet in an actual relationship with those men. And once they were in a relationship and expected those men to treat them accordingly, those men couldn't handle it and so looked for the next chick who they could have fun with until she figured out he's a jerk.