Instead, you should wonder about your ex: why would she knowingly go back to a cheating, backstabbing and unstable guy? Is it because she has abandonment issues? Because she is the kind of person that feels attracted to a "pseudo-bad-boy" kinda guy?

She seems to be pretty impulsive, taking important decisions based on temporary feelings. Either there something else than "i missed him/you" or she's prone to irrational impulsive behaviour. In both cases she doesn't sound like a very sentimentally mature person if you ask me. Is that really what you want as a partner, someone that acts on context rather than content?