Will finding the right person fix him?

Breakups are painful, but more so if you are anxiously preoccupied.

While I understand that you are hurt, you are perpetuating the dysfunction that allowed you to co-create the toxic relationship you had with him. It is only because you are insecure that you long for anything concerning this man . He was unfaithful, and the relationship was unstable. You may look back and exaggerate or minimize the good or the bad,, but the fact remains that a substantial amount of time has passed and you have not been able to let go

What you are doing here is providing credibility to what he may be saying about you-
She can't move on.
She still wants me.
She is miserable and has her own issues to deal with but she is obsessed with me so she will just stay stuck.

Rather than pose a question like this, and have everyone speculate about what is going on with him (pure speculation) , which in the end does absolutely nothing to move you forward in your own mental and emotional health... wouldn't it be more beneficial to write and question about how to actually address the lingering preoccupation with him?

Of course, you need to do what you find is helpful. I'm only suggesting that this might not be it.

I had an ex that was awful to me, I left him. he ended up with a woman 20 years younger and everyone swore they would fizzle as soon as she found out how he is. He is actually the same- but her dysfunction meshes perfectly with his and they have been together over a decade.

However; that's got nothing to do with my life. I moved on from him and got healthier, my relationships got healthier , my life is successful and I learned a lot of lessons from the terrible relationship I entered into with him due to my own unhealthy relating style.

Many years later, I still grow. He's actually old and unhealthy and unattractive now, has considerable financial difficulty, and It doesn't matter. I've reaped the benefit of my dedication to growth in my own life, and my experience is that taking active responsibility for my own emotional health keeps me too busy to spin in circles about what someone else is or isn't doing.

He was bad for you. Start over, clean break, focus on letting g go instead of speculating about him and his life. It's only keeping you down, the relief you may feel over thoughts of his failure won't help you one iota in the end- and the real truth is that you could find yourself repeating the pattern win someone new instead of fixing it. Most of us don't have just one failed relationship, it's a series. Limit that as much as you can. it sucks.