In my experience, people just do not change like that, not ever. I mean say someone had some huge traumatic event, like a horrible accident and they found religion. Well that's one thing. But I suspect those events are rare.
I don't know why I'm selling myself a different narrative that I was collateral damage in him becoming a better partner for someone else. Probably because he moved on so fast and I am still heartbroken, scared and trying to process.
Unfortunately it takes time to heal from these things. But you will get to better days and you will laugh and smile (and probably wonder what you ever saw in him), but again, that takes time. When you start to change your narrative, you will probably find yourself getting angry (rather than sad).
A better partner for someone else. No, he will do the same to her as he has done historically. You may think you made a difference in his behaviours, he probably told you that, he may even have believed it at the time. But, no one can fight their nature without a huge commitment to change (and probably a lot of professional help).