I understand -- I've been there, too. I have a dear friend that is seriously bi-polar. When she's 'up' she is so much fun to be around! But when she's down, OMG -- she can be depressed like that for months at a stretch. She brought out the best of my rescuing tendencies and I must have loaned her $2-3000 dollars over the years -- of course never a penny was re-paid. That was part of her issue, of course. She would over-spend and then get depressed about her financial state. She filed bankruptcy twice, allowed her house to go into foreclosure (which then had to be demolished because of the condition), etc. I woke up and realized that she was leaning on me way too heavily and I had my own problems to deal with. When she "emancipated" her 16 yr old daughter and allowed her to move to another city alone, I said, "That's IT. I can't watch this potential train wreck anymore." It was just causing me too much heartache. I walked away.
I run into her sometimes. Last time she told me she's recently been thru a series of 13 "shock treatments" to get her depression under control, she lives with her also-seriously-bi-polar mother, and her daughter is now 20 and an alcoholic.
There was nothing more I could do to help her and being a part of her life was tearing me apart. I was WAY too enmeshed. I could probably handle it better now, but I choose not to renew that friendship although I do miss her sometimes.