How do I move on?

[WellThisIsABitRubbish]

Im after any advice you have to help me not give a crap, I'm sure there are many of you going through similar or have come through the other side.
2.5 years ago I found out my partner of 13 years was cheating on me with someone from his work, he promised it wouldn't happen again, it was a one off, didn't mean anything, the usual script, I wanted to believe him so gave it another go.
I threw myself into our relationship and I guess without realising I was doing the "pick me" dance, cut a long story short pretty much every 3 months I would find evidence that it had happened again, after 9 months I finally saw the light and told him to leave, we then co parented successfully for a year and all was ok, we generally get on so would take the children out, he would stay over etc, he's a good dad who is always there for me and our children when I need him.

This week I have found out they are now in a relationship and it's knocked me back to where I was when I first found out, I don't want to be with him but it still feels pretty crap, it's stressing me out thinking that the person who played a part in wrecking my family could become part of my children's lives, I've also told him there's no way I can carry on as we were now I know he's with her.

I was fully prepared that he would go back as he's proved many times he's weak so I'm not sure why I feel like this.
Any wise words to help me move on?

Well done for leaving, it must have been hard. Remember you ended the relationship because you didn't want to be treated like this and you deserve better.

What you need to focus on now is your children and how you both co parent? Have you asked him how serious his new relationship is? If it's serious, whether you like it or not this woman is likely to be in your children's lives. Have you met her or have any indication of how she will be with the children? That's the most important thing, not who she is or how his relationship with her came about (despite how upsetting that is).

Thankyou for replying @Swimmingwiththebees
Any conversations I try and have with him are met with “but you ended it” like I had a choice he will do everything possible to avoid talking about it as the guilt he feels is massive, I havnt had and don’t plan to have anything to do with her, our children are fairly young so they are completely unaware which is how I want it to stay, he says he understands why I feel like like this and it’s not on the agenda anytime soon I’m just trying to prepare myself for when the time comes.

Blimey how awful for you, but you appear to be handling yourself so well. Well done you.

Just give yourself time. No big decisions. Focus on yourself and the kids for now.

Thankyou I regularly feel like I’m losing the plot so that’s kind of you to say, I feel so resentful and angry that he’s going about his life and I’m here picking up the pieces on my own, it’s just so unfair.
You are right I need to put him to one side and focus on our children, shame he didn’t do the same.