Dealing with Negative People

I have a friend that has been through a lot of awful things in life and is very negative about everything around her. She severely lacks social awareness and cues from others.

Often, I find her going down rabbit holes of stories about people I don’t even know and going into detail about things I just personally don’t want to waste any of the limited time I have on this earth listening to. If we are on the phone I will tell her I have to go and we will go through several rounds of her not taking my very clear cue of, “Ok, I have to go I’ll talk to you tomorrow,” before finally hanging up the phone, to the point where I have stopped answering her calls.

I need help with my intentional model, I am struggling. I care about her very much, our kids are very close and I am afraid to share my feelings with her because I’m afraid she will go into a deep depression over it. She is very depressed and heavily medicated, and is a single mom. I worry about how my honesty may affect her and therefore her daughter.

One of my unintentional models is:
C – Friend talking
T – this is a waste of my time
F – anxious
A – deliberating over how to get out of the conversation
R – wasting my time deliberating over how to get out of the conversation

One intentional models I’ve attempted is:
C – Friend talking
T – I don’t let other people’s negativity affect my positivity
F – Strong, courageous
A – Set boundaries (e.g. tell her I am a positive person and am happy to hear her talk about things that directly affect her but need to draw the line at stories about her neighbor’s cousins)
R – maintain positive mentality

I’m just not sure I can get there yet. Part of me wants to say that the result I want is to just not have to deal with her, but I don’t necessarily want to end the friendship either. Please help!