You can’t fix your mother. If she wants to fix herself, you can help her. You can’t make her want to be fixed.
There is a huge stigma about mental illness in most of the world. Far too many people feel it is a sign of weakness and a moral failure. You can’t control your own mind and feelings. I wonder what kind of messages you’ve gotten in your family about this? Is it all about personal responsibility? Is it scorn for those with mental problems? Sounds like it, if she is not sympathetic about her mother.
One thing that might help is information. Bipolar and other mental disorders are, in fact, illnesses just like cancer or AIDs or kidney disease. It has to do with brain chemistry. There is a little you can do about brain chemistry via self-help means. You can get a lot of regular exercise. You can sleep regularly and properly. You can help others. You can have a regular schedule. You can have responsibilities.
You could offer to run with her every other day. You could ask her to join you in volunteering somewhere. You could urge her to sleep properly, or set a rule to turn off the TV at 11 every night. You could ask her to help you with your schedule. If she bites on any of these things, it might help. But there’s a limit. After that, it’s medication. But you have to comply with medications, and if you don’t believe you are ill, you won’t do that.
Does she get depressions? Manias? Does go on spending sprees? Does she have times where she has a million great ideas, but doesn’t follow through on any of them? Does she have the remains of a hundred projects in her office? Has she always worked on her kids to help them with “self-improvement” projects? Does she fall into depressions where she can’t get out of bed? Where she wants to die? Where she acts angrily in response to the slightest thing? Are there periods of time when no one around her can do anything right?
If she gets an annual checkup, a good physician will ask about these things, and if they see a problem, they might suggest a psych consult, or even prescribe meds that might help. On your own, you can only do these oblique lifestyle things. I suppose you, being the youngest, feel a great deal of responsibility for her. This is not a bad thing. However, if she feels bad about herself, she is going to make it extremely difficult for you to help her. She will push you away at every turn. If you go away, this will confirm her opinion that she is shit.
However, there is only so much you can do without ruining your own life. I hate to say this, because the only reason I was diagnosed and helped with my bipolar is because my wife gave me support at every turn, even though I tried my almost very best to push her away. I knew exactly where the boundary was, though, and I didn’t quite push hard enough to make her get rid of me, but I could have. I’m glad I didn’t. Even though I was the worst husband and father in the world, and even though the only thing I deserved was homelessness and death, I had enough to know I didn’t really, truly want those things. I wanted to be saved. Not everyone with bipolar is as self-aware as I am. Not everyone can see what they are doing. Not everyone can moderate it.
As a sufferer, my feeling is that I hope you stick it out with her. As a person who knows how difficult it is, and values your life, too, I know there is only so much you can take without ruining your own life. I hope you don’t go there. I hope you get out before she makes you crazy, too. But I hope she decides she wants to get better before she drives you away.