Sometimes people enter into a second marriage to try and "fix" what they perceived was wrong with the first one (or trying to fix again what was wrong about their childhood). I know firsthand that this is a bad reason to marry someone. Like me, you may discover that the first marriage was not quite as bad as you once thought it was and your first spouse was much closer to being a person that was "right" for you than his/her replacement. Though we may find the tone of her post oft-putting, Cassandra still deserves compassion. It is easy to tell someone that if they are unhappy they should just get a divorce. But that is asking Cassandra to pay a heavy price, namely the economic and psychological consequences of becoming a single mother of two children. Cassandra, you are right in making your kids the priority. Sometimes staying together "for the sake of the children" really is the right thing to do (despite what our self-indulgent culture might say). But you have to ask yourself if the toll your marriage is exacting upon damaging your ability to be a good mother to your children. Trust me, they already know, at least on a unconscious level, that you are very unhappy.
I will be praying for you Cassandra.