I am really fed up, should I call it a day on a friendship?

I have been in your shoes. Sounds like major anxiety. Do you feel you can tell her that you love her BUT that you are worried about her anxiety? I would let her know how it is affecting your life and that you will have to cut back the time you are able to commit to her phone calls. In my situation, the person with the mental heath problem is my sister. I could never shut the door on our relationship BUT I can draw lines to protect my sanity. You really need to take care of yourself first. That isn't selfish, that's self preservation. Good luck!