Hi , as someone who's gone through the disability process, I have some experience here. As far as knowing when it's time to stop, I think that's definitely an individual thing but something you should at least start planning for (it happened suddenly to me and I wish I had had given it more thought to it before it did). If your symptoms are impacting your ability to work, then maybe it's time (I am guessing that being an RN with brain fog could be potentially dangerous as well as difficult).
Regarding the disability process, I highly recommend getting a disability lawyer (it is surprisingly affordable). It dramatically increases your chances of approval. Even having them help fill out the forms can make a huge difference as they understand the terminology to use and the best way to fill out the forms to have the best success.
As far as disability helping, well, it's difficult, it isn't much money, but it's the only option for many of us.