To ask if this is a rebound

[Klepp]

My husband and I were together for 20 years. 5 months after the relationship he has met somebody new and introduced her to the kids etc. it was my decision to end it but I can’t even think about another relationship. Did he care about me at all?

More than you cared about him by the sound of it

What?? What a strange reply based on nothing!

Based on this:

"it was my decision to end it"

Men move on FAST and mostly cannot be alone

Some people get over things quickly. My exh moved out - after 20 years - and I had met dp within a few months. I’ve now been with dp for 5 years. With your ex it might be rebound, it might be a genuine connection, you won’t know for a couple of years.

Regardless of who ended it a person moving on quickly is a bitter pill to swallow for the other person so your feelings are natural and understandable. That said, he’s not done anything wrong and was free to get into a new relationship. You are going to have to learn to accept this (or become very good at pretending) so it doesn’t make your DCs feel anymore weird or uncomfortable about their dad being with someone new.
20 years together and children, of course he cared at some point and probably always will to a degree. But you don't know the point at which he checked out of the marriage. It could have been years before you ended it. I doubt you ended it on a whim, so you must have been unhappy and had irreconcilable differences. Unhappiness in a marriage is rarely a one-way street and he was feeling it too, even if he never did anything about it.
People process grief in different ways and in different time frames though, so even though he’s with someone new so soon, it doesn’t mean he has processed the divorce. In my experience men often have a delayed reaction or distract themselves from their feelings with rebounds etc. and the emotions come later. Usually once their ex has fully moved on and is with someone new and they start acting weird.
For now focus on your own life and processing your feelings about the divorce and learning to be on your own. Time will tell whether your ex has really moved on so quickly, but that isn’t really your business anymore so it’s not worth getting caught up on or giving it too much head space.

My ex did similar, OP, though he did try to hide it from the children for a while.

What I have thought for a while now, is that he was such a poor husband that I don’t really want to pursue a new relationship. I want to learn to live on my own.

I guess our marriage wasn’t so rubbish for him as he’s very quickly put a substitute in place.

It’s a bit galling to feel you’re so easily replaceable, but you did leave him so he obviously didn’t make enough effort to treat you well. I do think mine cared, but again, not enough.

It's not a reflection on you or your relationship with him. Some men are just rubbish at being alone and will jump into a relationship with the first woman that shows an interest. Don't let it bother you - you didn't want to be with him so why care what he's doing now?

A friend’s husband moved on really quickly after she died and I was astonished at the speed, but as her ds said to me, he’d looked after her for ages, he was ready to live a little and honestly, this made me realise how bad his quality of life must have been for quite some time (she had a long illness). If you chose to end it, maybe he was blindsided and wants validation/comfort/reassurance. Who knows? Are you, in fact, seeking validation that it’s meaningless that he’s moved on so quickly?