What do you do when someone needs your support constantly but they bring you down with them?

A friend of mine has a bad mental state, but she won’t/cant get professional help and she comes to me to vent and complain. But nothing I say makes a difference. It’s exhausting and a little bit frustrating because I want to be there for her and I want to help but it gets me down too. I feel awful that I feel that way about it but I can’t help it.

If you have any advice on what to say to her or how to handle the situation any better I am all ears