First and foremost...Keep your patience. Don't say or do anything you'll regret later. Remove frustration and anger from your emotional pack while in her presence. Save it for later.
Mom wasn't "losing it" at the end, but there was a lot of frustration in dealing with so many things, and yes, dealing with dear old mom could sometimes be frustrating and just hard in general. But she never saw that from me, and for that I am very happy. I tried (and do now) look at it as a privilege to be there for her every need in her last year. And am incredibly happy I have no regrets regarding how I interacted with her. I know others who did not do so well, lost their cool from time to time, and they carry the guilt of that heavily.
DO consider getting someone in to help you so you can get a break now and then. It is very emotionally, mentally, and even physically draining.
Is she a candidate for palliative care yet? These people are solely focused on her and her well being. It is a "one stop shop" for getting her health care managed, home visits, and they will also offer counseling and support for both her and her care takers.
Trust me, you will need breaks from this. Try to arrange for that. If you are going to be your "best" for her when with her, you will need time to take care of yourself and de-stress from what you are facing.
If she's in need of medical care....look at palliative. If she is not in dire straits medically, get someone in on a part time basis so you and the W get a break.
