I get why this is upsetting to hear OP. My Mum was 60 when she died, and in the last few months of her illness, she'd had enough. She hated the pain, she hated the uncertainty, she hated the fact she needed so much help, the lack of independence. She hated the fact she was a burden on those she loved.
Some days, she hated the fact she was still alive. Some days, she saw it as a blessing, she got to spend time with me, her grandkids etc, some days she had new experiences, enjoyed being alive. But some days were shit.
On the shit days, I got the phone call. She needed to vent, and I was the stoic one. She had to tell someone, and who was she going to tell aside from my stepdad, my brother, me or her best friends.
She judged me the best able to cope with that, and I probably was, but by god it was hard. But I wouldn't have given it up, because this was how I could help. I couldn't take away her pain, or frustration, or fear, but I could listen. And when it threatened my mental health, I spread it outwards, I vented to my DP, or my friends.
@LindorDoubleChoc Your mother is telling you this because she needs to. Its a horrible, terrible honour that she feels she can tell you this stuff. See it for what it is, and then do what you need to to cope.