I had a "best friend" like that for many years. I considered her 'like a sister" and I was the friend that was "always there for her". All of her other friends and family admired me for being so loyal. However- I finally realized that she was wreaking havoc in my life. I had to admit that I had outgrown her and that she was so dysfunctional and self centered that she was not really contributing anything to our friendship or to me. In fact, she began to sabotage my primary relationship and that is when I had my wake-up call. I had become her rescuer. It was one sided and had been for a while. I ended the relationship. I still think of her and I miss her, but I do not regret ended the friendship. I have moved on and I have made new friends with healthier people who respect me.
The key to life is in being fully engaged and peacefully detached simultaneously and authentically in each moment.