Able to maintain intact much of her personality throughout her ordeal, mom put on a happy face for just about everyone who visited, especially her grandchildren, even as her brain shrunk, even as she lost control of her own body. But she & I lived honestly with each other and so one night, towards the end, she struggled to construct a sentence confiding in me about her experience that "this is horrendous" and she asked me, pointedly, "what can you do?" I said, mom, there is nothing that I can do. All I can do is keep you as safe and as comfortable as possible. I don't know what else I can do. Mom looked at me and said "I don't know what I can do either."
She wanted to die. She'd had enough. But she missed her window of opportunity and I could not kill my mother nor would she, in a nondemented state, ask that of me. She had seen this coming. She volunteered herself for testing and experimental treatments for almost a decade before that became too much for her to handle. I was intimately involved with her struggle from very early on. She held out hope for a cure. She loved life and did not want to leave life early nor to leave me at all.
Though many of my relatives live healthy lives into their 90s, I've just one bloodline which came down to my mom who died early of Alzheimer's. I think to myself now that I'd prefer dying by my own hand than experience my brain deteriorate so horribly, especially as I've put a life's worth of work into expanding my own consciousness. As far as I'm concerned, when I'm done learning, I'm done living.
Though we can ask healthcare providers not to take extreme measures to keep us alive if we are near death, because it is unacceptable for advanced directives to instruct another person to end our lives early--if for no other moral reason than to avoid laying that guilt on them--we never quite know how the end of our lives will play out. You could suddenly find yourself stroked into paralysis, unable to reach that rat poison you've been stocking up on. Or where you think you have the will one day, dementia might rob you of it the next.
Also, being in your 20s now, you don't know how you will feel about life in another 20 or 60 years. So you might think you would take certain actions, but you won't know until you get there, even if you are able.
Love your grandma while you have her here. That's all that matters.