How to tell friend they are being clingy ?

How do you tell a good friend that they are being clingy ? I have a friend that has been diagnosed with bipolar and schizophrenia when she was a teenager. She has had many traumatic events in her life and she hasn’t been able to keep steady friendships over the years. I’ve been the friend that would be the shoulder she can lean on and will be cautious what I say and do because I know she is sensitive and will get hurt easily. HOWEVER, it’s taking a toll on me. I work full time and in school for my masters degree so I’m busy. She text me all day long and calls every single day at the moment I get off of work. Although, I’m tired I still try to message her and talk to her on the phone but a lot of times the last thing I want to do is have a hour long conversation when I get off of work, I want to relax and enjoy me time. She tells me that I’m the only person she has to talk to because her family isn’t there for her and her friendships in the past hasn’t lasted because they would get fed up and end the friendship. It’s like I feel obligated to be there for her. She is a good friend and I like being her friend but it’s becoming too clingy. Any advise on how to talk to her about this and still keep the friendship ?