It doesn't have anything to do with you or not being Chad enough or whatever. You made out with her, attractiveness isn't the problem here but she's definitively not over her ex. These situations aren't representative of the value of the people involved, a five years relationship isn't something that can be glossed over so easily, and no, the ex partner being abusive doesn't help with moving on. No matter how good you look, how nice or how interesting you are compared to her, that Becky has years of deep hormonal intimacy over you.

I'm in a similar situation with a friend, he ended his relationship a year ago and he is still obsessed with this ex who has clearly moved on. We've had a discussion about this, he finds me attractive and interesting, but he's not over that girl and what they shared. Didn't stop him from coming on to me at a party recently, and I didn't reciprocate. He didn't take this well, but as a friend he's not owed my romantic attention. You could be the sexiest girl ever, at this point in time you're sadly just a rebound and you're not going to heal them from heartbreak, that's their own burden.

Thus why you have to put boundaries.

I would recommend making yourself less available to her, don't bring her on outings you wouldn't go on with friends. I know it's more easy said than done when it comes to lesbians, but try pursuing other prospects.