Wow, it's a tough spot you are in. I know how hard this position is. You already feel like you are a single paretent, but not ompletely. I don't really know what to tell you about staying or going. You know your situation & feelings more then anyone & know what would be best for you & your girls. I am sure that if she started trying to take care of herself & her issues then you would stay. It also sounds like you are already feeling guilty for leaving & you haven't even done it yet. I know how you feel, with my wifes condition (BPII, rapid cycleing, severe depression, psychosis)there is no way she could make it on her own. I am having to do everything as well. Cooking, cleaning, laundry, taking are of our son, plus being her cheoffur. The only thing she really does for herself is personal hygene. She says she is "letting me" do all this stuff since I am out of work & it will help me to fight the boredome, but I don't buy that.
Maybe if you went to her & told her that you need her help with things she may snap out of it & pitch in. Sometimes the feeling of uselessness feeds the depression, so make her feel like she's not. The underlying thing is, she has to want to. Until she does, then you probably won't see any progress.
Hope I have been at least a little helpful;
It really hurts to see someone you care about this way. If staying for the girls sake is the only thing keeping you there, then you know the answers already. Is it really benifitting the kids to be around her, it doesn't really sound like she is bringing anything to the table. Kids need their mother, no question, but she isn't really filling that need for them. You have to decide what is best for you & yours. Maybe the thought of losing everything would be a wakeup call for her to try to get herself back up on her feet, because until she decides she wants to do it herself, there is nothing else you can do.
I feel for you, & wish you the best. Stay in touch & lean on the folks here anytime.