Hi SB. Having to raise a little baby and two daughters must be difficult, especially with a sparse husband around. Stay strong, and keep positive for your own sanity and your childrens'.
It sounds like your H is in a very dark place, and dealing with a lot of his own emotions. It might be better that you're not currently involved with him, as you can't allow his depression to effect you. Don't look at what he's doing as replacing you, but as him acting out on his own emotional downfalls.
With the divorce coming up you're probably feeling closer to finalizing the marriage, with less hope. Try not to think of it in those terms. Keep yourself surrounded with family and friends. Maybe have grandma take care of the baby and go out and treat yourself. Stay happy even if you don't feel like it!
