Looking for support with abusive but mentally ill friend

I recently cut off a close friend after almost 15 years. I love her so much, but our friendship was abusive. The tough part is that she is mentally ill. This mental illness is taking over her life and wreaking havoc in her relationships. I've begged for years for her to get help, but she will not seek treatment and it has just become worse and worse. She recently became verbally abusive towards me, and it was then that I cut off our friendship. I told her I could not be in her life anymore unless she got help. I now recognize that in many ways she has been emotionally abusive and controlling for years even though the verbal abuse just started. Therefore I realize I did what was best for me, but I also realize she is hurting. I feel like a terrible friend and like I am abandoning her but I also feel like it was killing me to watch her spiral downward without being able to do anything and her behavior is not fair. My therapist is encouraging me to keep her number blocked and not to contact her. But I am really struggling with that right now because it does not feel like the right thing to do because I am abandoning a friend who is in need. Does anyone have any advice or insight they feel could help? Anything is appreciated.