Rachelle,
Please, Please stay strong. I know now you are confused, sad and lonely but please get on your knees and ask for God to keep you strong. You can't go through this on your own and if you don't turn to God you may feel the void with the wrong things.
Is this man the father of your child?
Just know that Heavenly Father is just, and if you're doing what you should be doing he will provide for you. Could it be that this guy wasn't meant for you? Even you mentioned that he's on mood stabalizers, is he Bipolar? He could be in a manic phase and may end up doing the same thing to this new women.
God could be protecting you from long painful path. It's hard to see beyond now. I'm going through a difficult time as well and everyday gets better. You may surprise yourself at how strong you actually are.