Codeine - what you said is dead on. Drained, your not giving yourself enough credit. You ARE stronger than you think. You are in therapy already, which is a great start. Boundaries are important but uncomfortable (at first). Everyone who you exert a boundary upon will resent you for a minute or two because the boundaries will be out of character for you. It will get easier and easier and soooo liberating. I bet it will become your addiction because everyone needs boundaries. Everyone.
Living with an addict or alchoholic takes a GREAT deal of strength you just need to turn that strength around and focus it on you and your children. Your girls will not respect you for staying with daddy because of finances and I know you don't want to set that kind of example for them. You want better for them right? You don't want them hooked up with someone like their father do you? Kid's learn by example and neither of you are setting good examples right now - him because he's using and disrespecting the family and you because your rolling over and taking it.
Work with your therapist. That is so good and try to get to a meeting. For some of us, stepping away is not an overnight decision. At least for me, I have to hear it over and over again before it clicks in my head and I act on it.