It is a pretty shit thing to happen, the first thing to help you along the way is to look at it this way. While he might be happy with her and his new life, as soon as this happens, there is always an opening for a new mistress.
Now along the way, people make decisions, and well, while it isn't one you would have liked to have made and it was kind of made for you.
One of the decisions you have made is to stay at home and look after the kids. The thing is you have made that decision, and you receive CSA payments to help you do that. So, if life is shit, why not change it and give him the kids and go back to work and start paying CSA to him? It is a fair comment, because if you take the male/female part out of it, both parents are equally important, and not one is more important than the other. I am expecting a lot of flack over this, being that this site is mainly female, and all men are bastards, but at the end of the day, not one parent is more important than the other, and their ability to look after a child while it may vary, I can show you just as many females that are bad mothers as there are males that are bad fathers.
What I always fail to understand is that it seems to be a god given right that the male pays and is expected to pay for everything, and they are bastards when they don't. So what if the roles were reversed?
I pay £670 a month in CSA, and don't even get to see my son. I also paid all the bills as well, including fuel (nearly £500 a month!), council tax, mortgages on her property, and pretty much everything, until one day she helped herself to a lot of money from the joint account which caused my mortgage to bounce on our joint property. This left me going backwards at £1000 a month, but I did it because I thought it was the right thing to do.
From where I stand, both sides have a financial contribution that needs to be made. I know looking after a child is a hard thing to do, but it is a decision you have made. I know that I would love to have changed roles, but I don't get to make that decision.
At the end of the day, it is a pretty shit situation, and well, don't envy it at all. The only thing I can offer is that not all men as bastards, and that you do finally meet someone who respects you for who you are, and doesn't shirk around on you. They are out there, you just have to find them.