LostDove 
I have only accepted recently the negative aspects of my relationship with my mother...
Accepting negative aspects of a relationship can be difficult. When we realise that a person whom we have a relationship with has a character or traits that are inconsistent of our expectations of them—then the disappointment can be difficult to feel.
[...] as many of you have probably felt there were times she also became the exact opposite.
Yes, with the BP in my life, there are times where I've experienced these two seemingly opposite personalities. Of course experiencing that inconsistency with a person over a long period of time can feel very taxing—we're struggling in a way to relate to someone consistently when
they're expressing themselves inconsistently.
I have become a parent in a sense to her & I feel it is my job to make sure she is taken care of despite all she has done against me...
You aren't alone. I think it's quite common for some adult children of BP's to feel like 'parentified' children. When I once expressed this to a friend of mine, she seemed to not have a clue what I was talking about (she came from a FOO where parent issues didn't seem present to her). So me too, I've felt alone because I think a lot of people can't relate—they don't have these experiences. I encourage you here that you aren't alone. Lots of people have gone through this. There are a lot of great ways to improve your life from here.
I have been in denial about her negative behavior for a long time, ultimately believing I could fix it or her... [...]
Me too, I thought I could fix more than one person for many years. When I let it go, thinking and feeling through things in as rational and as moral a manner as possible, it improved my quality of life tremendously. So I share that hope with you too.
[...] maybe the same or similar situation I have...I haven't put myself first in a long time & I know I must to gain my mental health back...
It may not feel like it now, and I haven't got advice on what you can do in this moment (keeping a theraputic relationship is a
very good thing)—but I share with you a bit of understanding that gave me comfort.
When I was 'adjusting' my relationships with the BP in my life, and other people that fit a similar bill—I did feel angry and sad. I appreciate that it can even feel depressing and being drained of strength. One thing I noticed was that the feeling was similar to not getting what I wanted from a relationship. I felt pain because I was saying goodbye to the image of what I had of this person. I seemed to derive a lot of 'good' things from that image. But it was a false image in the sense that it didn't fit the reality of what the person more accurately was. My thinking went like this—I wouldn't have the relationship I thought I had with the fake healthy person I
wished I had. Saying goodbye to wishes is of course painful.
Instead—looking forward—I'd have a relationship with a BP/unhealthy person. In that case, it's a much better mind's eye view/position to approach the relationship because (1) I wasn't going to expect things the other person couldn't give and (2) I wasn't going to be as vulnerable to manipulation from a person of whom I had a false image of. I found comfort in that, and I share that with you.
So it's a bit like an infant teething. In the short term, it feels really very crumby. But in the long term, the infant gets to use their teeth.
