I dont know your moms prognosis but will relate a story about my moms fight.
When my moms cancer was found it was already terminal. she was given 6 months without chemo and a year with chemo. of course she jumped at the chemo for that precious time. In retrospect she should have passed on the chemo. every week she got a treatment she would be terribly sick for several days. Delusional at times. She had to endure countless Dr's visits and blood draws and days spent at the infusion center. I did get to spend a lot of time with her but it wasn't quality time. In the end we really only had her for 6 months, the other 6 were wasted on all the chemo hoops.
Afterwards I was pretty hard on myself and blamed myself for encouraging her to fight it. I felt that i was just prolonging her pain. Took a while to get past that. I realize now that it was the best decision I could make with the knowledge that I had at the time.
I've been down this road before, PM me if you have questions or just need to talk.