I'm so sorry, Catgirl26.
(((((Catgirl26)))))
You have every right and responsibility to make sure that your children are cared for as well as possible. It sounds like your perception of the new woman is coloured by reasonable concerns about how your children will fare if a divorce occurs, she becomes their stepmother, and your financial connections with your husband are dissolved.
But it sounds like your pain comes from from more than concerns about your children.

It sounds like you still have a strong emotional connection to him, and it is therefore so painful to realise that he might have found someone to move on with.
I'm so sorry for your pain, Catgirl26.