I assume that if your mother is 80, you are a grown woman and my best advice to you is to recognize your own age and position in life and remind yourself that you are an adult who deserves to be treated as such. You are no longer a child living under your mother's roof where she has complete and total control over you. You no longer have to try to earn her love, walk on eggshells, or give into her endless demands despite poor treatment.
Look at any one in your social or professional circle who you respect and ask yourself, "What would they do?"
When it comes to boundaries, the thing you have to accept is that a narc will never, ever, ever honor your boundaries. They will push, manipulate, guilt trip, give the silent treatment, and basically use up all the tricks in their toolbox to get their way. A narc only cares about their own needs--so why are you afraid to care for your own?
The boundaries you set will be for yourself, not for your mother. You have to accept that you are never going to be able to reason with this person. A true narc will never be able self-reflect, take accountability, or sadly, change their ways UNLESS it is part of a manipulative tactic that is often short-lived until they get their way.
You have to be strong and remind yourself that she has trained you to feel guilty. Normal, healthy family bonds are not built on fear, obligation, and guilt. They are built on mutual respect, compassion, understanding, and love. Ask yourself what your mother has ever done to make this relationship work. Ask yourself if she has ever once felt guilty for how she treats you.
You need to set yourself free of this mindset of how others perceive you. That is all tied in to the way you were trained to measure your worth based on how others responded to you. You had to earn love, and you have seldom been rewarded for it, which has just make you work harder to try to earn it.
If you can't be objective in the moment, I'd write down a list of all the good things you have done for your mother (or others) as a reminder to yourself.
So start with telling her that you can't bring her to the appointment next week due to a scheduling conflict. If she gets nasty, calmly say, "I have don x, y, and z for you and do not deserve to be spoken to this way. If you are insist on talking to me like this, I will end the call." And make good on your word.
I'd also check out the books and FB page for Dr. Sherrie Campbell. She's an expert with this type of situation.