I posted this on my other support group site, but I need all the advice I can get, so I'm posting this here as well. I hope I don't offend anyone who is a member of both sites with a repeat post.

I'm hoping to that some of you can relate to this, as my husband and closest friend just don't get it because they're not bipolar.

I have this friend that I've known for 13 years. She has been one of my closest friends through most of that time. She has BPD- Borderline Personality Disorder, so she puts on facades that aren't really her to fit in with the people around her. If I say that I don't like something she's doing, she'll change it to keep me as a friend, even if it isn't true to her personality, and eventually will relapse back into the harmful behavior.

Lately she's been engaging in destructive behavior. Last night she posted on craigslist looking for a one night stand even though she has contracted STDs from her promiscuous behavior. She won't take her medication regularly, but then complains about being depressed and unstable. She wanted to go back to school, but blew the money for it on junk for her house. She started compulsively lying again.

I have been with her before, supporting her through her rough patches, but this time I don't know if I have it in me to help pick up the pieces when she eventually wakes up and realizes the mess she's made of her life. She won't listen to me, and I've accepted the fact that I can't change her, she has to want to change herself first. I had to end our relationship before when she refused to take care of herself, but we repaired our friendship after a year of not talking. This has her scared that I will leave her again. She has almost no real friends besides me.

This is the part my family does not get, and I'm hoping some of you might. My husband and closest friend believes that she is not good for me, as she is not a very good person at heart (she lies, used to shoplift, is not very compassionate, is self-centered, inconsiderate, and is often rude)
However, despite all her many flaws, she loves me and has been extremely supportive as I struggle with bipolar disorder. I need all the support I can get some days, and I don't know what I would do without her. We've been through a lot together, and I view her as a sister, although a dysfunctional one. I don't want to stop being her friend, but I can't keep exposing myself to her negativity and rude comments (she occasionally says comments that demean me and can be overly critical, partially due to her mental issues- she has no tact), and I can't witness another roller-coaster of self-destructive behavior.

I'm extremely shy and I do not make friends easily. I depend on my friends and family to help me through the rough patches of this disorder, and if I stop talking to her until she straightens herself out, I will be missing a large chunk of my support. But is it healthy support?

My closest friend suggested I give her an ultimatum- "straighten up or I can't be around you anymore." I know if I presented her with that, she'd put on a facade to comply, but in reality she won't change because she doesn't want to. I can't help her, she has to help herself.

I guess I said all that to ask this: Should I continue a friendship with her? Is this a toxic relationship that is actually harming me? Any advice for dealing with this mess?
Thanks for listening/reading this long post. I'm done now.