Hi OP, I hope you're still reading. I do have some sympathy for you and can understand your feelings. It's quite a common scenario and I know of others who had a change of heart when their ex met someone else, so your feelings are not that strange or unusual.
However, just because you have feelings doesn't mean they are right.
In fact I often think their only function is to hurt and confuse us! The challenge is to recognise this and start to move forward.
I have an abusive ex I broke up with years ago - even though he begged me not to. (I realise now that he didn't love me, he just didn't want to be on his own.) At the time I was SO relieved to be out of it. But years later I was plagued with regrets and longing because we had a lot of good times too.
I think it's because I never met anyone else and was lonely. Because the ex is not there in your life anymore you conveniently forget the bad times and remember the good times.
BUT.... I know it would never work if we were to get back together. He knows it too - about a year ago he told me it was "too late", which upset me at the time but he is right.
What helped me to move forward was coming across a list I'd written when deciding whether to break up, of all the reasons to stay or go - and some ugly incidents I'd forgotten about. It reminded me of the REALITY - and that longing and wishing will not change someone.
How do you know your ex has changed? You are not living with him anymore to truly know - of course he loves seeing his kids now because he gets to be the Disney dad and doesn't have the reality of dealing with them day in, day out - feeding, sleepless nights, preparing their lunchboxes/evening meals, cleaning up after them, playing with them, reading them stories, disciplining them etc etc.
I do believe on the whole (though I do know of one or two exceptions) that people don't change much and were you to get a second chance, he would revert to type.
His current gf will at some point notice his laziness - if they are living together she will be the one picking up after him and doing the chores. Some women are willing to settle for this. It's not a life I could live and you know it's not a life you could live either. Please remember this.
Remember also that actions speak louder than words. He is telling you with his actions what his real intentions are - and those intentions are not in your favour - and that is no bad thing, you will come to see - you're just not "there" yet, but you will get there.
Oprah says we are all at where we're supposed to be. What you are going through now is an intense period of personal growth, when life is difficult and painful. When life is good we are not undergoing personal growth because we're not having to question anything or look deep inside ourselves to think about where we went wrong and how we can do better next time.
Personal growth feels like shit, but it is actually what is best for us. Change is ultimately for the best, although we often resist it.
It is OK still to love someone who is not right for you and who chooses not to be with you. The definition of love, it has been said, is to put the other person's needs and wishes before yours, even if his wish is not to be with you. Do you love him enough to honour that?
For your own peace of mind, try to distance yourself from him as much as you possibly can, given that he still sees his kids. Don't engage in conversations with him, don't hang around when he picks up the kids. The more you can disengage, the faster you will move on from him. It's like a "he-tox".
I have found that counselling really helps, so get yourself some counselling to put your recovery on fast forward. Onwards and upwards, OP, right?