That's really a rough situation you are caught in. Your friend seems to feel like she can't get the help she needs from anyone else, and doctors haven't helped her, but you're struggling too and her demands are more than you can healthily give. And you don't want to make her feel abandoned like other people have probably done time and time again in her past.

Both of you seem to have difficulty setting boundaries. You can't really do much to change hers, but you can work on firming up your own boundaries and just letting her know that there are limits to what you can do for her. It isn't a rejection, but you won't be capable of helping her if you don't take care of yourself.

It's frustrating to have been to doctors and therapists and not feel helped. I know because it took me several tries to find someone who helped me much too. But good ones are out there. She isn't going to get better by clinging to someone else who is struggling and who isn't trained to help her, and it isn't fair to you to have those expectations placed on you. You can be there for social support, but if she chooses not to look elsewhere when you have given all that you can reasonably give, then she is making a choice that you can't be responsible for, and you're not doing either of you a service by pushing yourself past your limits to try to meet her demands.