I hope Jack does change. but I kind of don't like this whole he'll change when he meets someone else or the right person attitude because it implies that somehow Sasha is partly to blame for his bad behavior and it takes the personal responsibility away from him. maybe she "enables" the cheating by not leaving him but is also quite possible that there's some emotional manipulation from him that makes her stay. (I've had friends who've been in those situations before which is why the doormat comments about wags always make me a little uncomfortable). and the drunk driving is not within her control. or at least, I'm not sure what she's supposed to do about it. people did this to kourtney kardashian when scott seemingly cleaned up his act for a 19-year-old. all of a sudden kourtney wasn't the right person to cure him of his addiction. and people went so far as to imply that she contributed to his addiction issues.
hopefully, jack will change, but it's really not his current partner or future partner's responsibility to ensure that happens. he cheats on sasha because he doesn't like her that much. logically, that makes sense and you'd assume he'd be a better partner to someone he actually likes. but what's stopping him from doing that now? I know people on here have said she has dirt on him. and honestly yeah that's probably true. but I also think it's a little naive to assume that the person with the most power in that relationship is anyone but Jack.