Living, i do understand what you're feeling. I think something that happens in this circumstance is that when they are responsible for the relationship on their own, without any assistance or buffering from you, it's not so rewarding. I know my kids tell me at least when we were together, I intervened to make things better. I was the buffer that softened his wrath.

It's also sad that your ex has found someone else so easily and I'm sure that hurts your children. i've given lots of thought to whether it would be good or bad for my ex to find another. In my case, my kids are little and it would probably be better. I hope your kids can at least remember the good times and be thankful for that. When the novelty of the gf wears off, he'll probably try to spend more time with them, don't you think?