At what point can a caregiver say "enough is enough" to verbal abuse even if there may be a medical explanation behind the behavior?

The idea of you uprooting yourself from such a distance when she won't even accept your help doesn't seem to make sense. Since this is personality change (she hasn't always treated you like this) I hope that you can retain some compassion for her. Yet, building resentment toward her for how she treats you will eat you up inside.
I'd suggest counseling so that you can get your feelings out in the open with a trained professional. You likely feel some guilt, yet you are human. You can only take so much.
I hope you don't cut yourself off completely from your mother's care. You may regret that once she's gone. You need to decide what you can manage yourself and what you need others to manage - and go from there.
All situations are unique. Some people may have been able to jump in and move so that they can take over all care needs. Most people can't leave jobs to do that so they need to hire some kind of help. Some need more help than others.
Work this out with a mental health professional for your own good. Love your mother but don't give up your sanity to care for her. Visit when you can take it and be with her when it works.

You really must consider set boundaries as to what you can do and a professional can help you sort through it.
Carol

Your mom is dying. Her meds, designed to keep her comfortable and, perhaps, slow the progression of the disease, are turning her into someone you don't know. Say hello to care giving. Nine times out of ten? That's what it's all about.

You shouldn't feel shame in being unable to deal with her. That's natural. What's NOT natural is NOT being able to deal with it and trying to care for her anyway. There are times when professional care giving is the perfect answer.

I'd say this is one of those times. Don't be so involved. Seems like mom lives independently, so she has some money. Use that money to get independent people in who can cajole her, humor her and, most of all, don't have the family dynamic in the equation that makes good-natured care giving impossible.

Instead of hands-on, become her advocate. Plan for the time NOW for when she needs AL or NH care. That's what you can do -- and that divorces you from the hands-on care that you find so painful.

Does she have years left to live with a brain tumor?

Have you considered hospice?

You said "uproot hundreds of miles". Do you not live near your mother?

Your resentment is understandable but we have to be careful of resentment. When we carry it around it turns into depression. Do what you can to guard against this. From what you wrote you might already be experiencing depression.

As for why your mom heaps abuse upon you doesn't matter. Ok, she has a brain tumor but does that mean that you're better equipped to handle her abuse? Like you said, how much does it matter that her behavior is a result of a brain tumor? You still have feelings, there's nothing wrong with your brain, and it hurts when someone we love treats us this way, brain tumor or no brain tumor.

Are you her primary caregiver? If so, that might have to change. Can your mom afford in-home care? You need help.

Keep an open mind about assisted living. It's not fair that your mom has a brain tumor but that's the reality and you may have to consider options you never would have considered in the past, like assisted living. Or even a nursing home. You can't take of her all by yourself. You're going to need help. And while well-meaning friends and family find it easy to say that it's the brain tumor that's making your mom behave that way it's not so easy when you're the one caring for her. And that makes you human.

Probably steroid rage. She needs the neuro-oncologist to prescribe meds for the depression too. My daughter had brain cancer, it's a real heart-rending experience. Eventually she will need a memory care nursing home, and that will save YOUR life.

Matie, I feel so desperately sorry for you. What you're going through is hateful. It really doesn't make any difference what is making it happen, or anything simplistic like "whose fault it is," it's still you who has to stand there and take it. It's just awful.

All I can tell you is what my cousin did. My aunt died nearly ten years ago of brain metastases after breast cancer. My aunt lived in London, and my cousin's whole life was (and still is) in Canada - husband with heart issues, children and grandchildren, interests, work, everything.

So, Rosie came over for one month, and lived with my aunt in her ALF. No two ways about it, they fought like cat and dog. Never an easy relationship, it became hell as my aunt's domineering character became destructive, self-destructive and hostile.

But that's what she had decided to do: to give her mother one month of her full care and attention, then draw a line. Now, I can't tell you whether she has any regrets about not turning back, or wishes she'd repeated the exercise later (it was another four months before my aunt died); but I know she does have the satisfaction - the closure, if you like - of having made a plan and carried it out.

You are in the same situation in that you know that your mother has a terminal illness which nothing you can do will change. You are in a different situation in that the person who is being destroyed by that illness is a mother you were close to, someone you could trust, turn to, rely on. But that mother, the one you knew, how would she feel about *your* being destroyed by her illness too?

Don't give up your job, don't wreck your life. Find out what the best care options are and leave your mother confidently in the hands of people who won't be having to mourn their patient,or listen to abuse coming out of the mouth of someone they love.

You're not "resorting to" anything, you're not giving up on your mother. She will be cared for, and you will stay involved in that care. But you've got to the point where she would be telling you to save yourself, wouldn't she? I second that.

Matie, of course your Mom will be uncooperative, belligerent, paranoid, and simply mean... she has cancer for goodness sake. I wasn't a happy camper when I was diagnosed with cancer, either. Cancer changes your whole life. One has to find a new normal to deal with this long and crazy journey. It won't be pleasant. So please give your Mom the benefit of doubt. And find someone to care for her that understands what she is going through.

And no one plans to get cancer. I know I didn't.

Avoiding hands-on caregiving and "outsourcing" is the way to save your sanity. Probably would work out to be better care for your mother because a little (or lot) of distance can mean you don't feel glued in to abuse (whether malicious or the result of disease/medications). You would feel less resentment and would be able to relate to her more lovingly. And don't feel that you don't have the right to resent abuse...of course you do. You recognize that in your mom's case it is not deliberate but part of her disease, but you can and should do what is possible to lessen your stress.

My mother has a brain tumor also with inflammation and she has always been mean to me and my father mostly. My mother lives with my father in our on law apartment. Since the tumor the nastiness has increased to me and my father while my sister and brother she is different to. It hurts. I have had to detach myself due to my own severe health problems. My mother in law is in a another state with lung cancer. My father went back to work part time to escape and my sister has filled in to give some emotional support.

I hear it all the time...oh, it's just their disease, don't take it personally. Really???? I am here 24/7 (except for when running errands & what have you). The people saying this to me cannot be around them (aunt & uncle) for 30 minutes without getting frustrated and angry at them. What the h***?

An AL with a memory care unit is the right place for her, she will progress to that and eventually to hospice. Illness aside, she did not plan for her old age, so you now have to make the plans for her. If she has assets they get used for her care, else she ends up on Medicare. You need to set her up and oversee her care, but you need to get back to your life. Do not wait until your leave expires to start the transition, you need business hours to see homes, lawyers, etc..

Remember you can be loving without being subservient.