I like the ones where its drugs. An addiction she can't control that gets wrapped up in the feelings and motions of sex. Maybe she even desperately wants to quit and go back to her love, but she knows deep down that she is a young, immature, undisciplined girl who can't resist something as devastating as drug addiction. So it becomes an NTR cycle of sex and drugs until she accepts her new reality. Or, she's so blissed out all the time on either sex or drugs that she literally does not think anymore- that's the best, like an intentional, voluntary, self-inflicted lobotomy because she's too horrified by her own behavior to face it. So when she shows her old lover her new face, its almost a power trip, so to say, "look at what I did to myself and you had no power to stop me!" While he's going "what the fucking hell happened?"
I also enjoy when the drugs, sex, and indulgent lifestyle is reflected in the girl gaining a HEALTHY amount of weight- not becoming obese like in fucking /d/ weight gain threads. But a little more curve, a bigger ass, a little tummy, a little hair. Makes her look womanly and even more unrecognizable.