I am so sorry you are in that situation and also so sorry that your friend is burdened with undiagnosed mental illness. You are a very good person to be trying to be someone she can lean on. That is a heavy burden for you and I can see that you would want to be able to do something other than tell her that you have to go and do something when she gets into an illness-driven monologue.

Sadly I don't have any really good advice for you. I think the way you are handling it is good even though not perfect. It is what I would do if I was in your shoes.

Perhaps others here with more knowledge, experience and wisdom will see your post and respond with things truly helpful to you. I admire you for what you are trying to do. You are someone very special, a person of dignity and moral stature.