My Mom threatened to kill herself if she has to stay in assisted living. What do I do?

My mom is 63 and has a malignant brain tumor. For 6 months post-diagnosis, she was ok, but she fell off the cliff in August and has needed lots of supervision and help with daily activities. My father is 76 years old with a heart condition, and he could not manage it alone. At first I got them in home aides, but my father found fault with all of them and eventually told the agency to get lost. I was forced travel back and forth hundreds of miles to take care of every emergency, and eventually I just took FMLA leave to take care of my mom for 8 weeks. When I was close to running out of FMLA, I asked my mom if she wanted me to quit my job, or if she wanted to go to assisted living in my town. She chose the latter. I drove my mom 400 miles to an assisted living place 10 minutes from my place.

The first two weeks were great. I was there everyday 3-4 hours after work, and we spent the weekends going on trips and out to eat. My mom is the only resident with someone visiting everyday. But my mom has lately been very difficult and no longer wants to stay there. The assisted living was supposed to be temporary until I could relocate my father as well, but he has made no indication of his willingness to come. He is panicking over his heart condition, which has gotten worse, and he is on Xanax and an antidepressant. I think if he moved, he wouldn't be any help and would just be worse. If I were to try to take care of her on my own, I would need a new place, because I am 27 years old and rent an apartment with roommates my age. I would also need a sitter or CNA to watch her. I did the math, and the money my parents are spending on her assisted living would cover the equivalent of 55 hours a week at $18/hour for a home aide, which would mean I could only go to work and one shopping trip a weekend. I would have zero time for myself or to see friends, and I'm convinced I would completely burn out in a few months between the stress of managing my mom's care by myself and the stress of my job.

I explained this to my mom, and she said she will kill herself if she can't live with me. I lost my cool and told her that she is manipulating me and isn't thankful for all I've done since her diagnosis, and all I continue to do, and how other residents have no visitors but have managed to be there for years. All of the old issues I had with my parents came boiling to the surface. Then I realized I'm screaming at my terminally ill mom who has brain cancer and no hair and can barely walk, and I wanted to die from the guilt. I feel like a monster.