Pants vs quality of life

My kitty. Smokey. Smokey bean. Bitten. Button. The littlest bitten bean. My heart. My limbs. The tether to which I grip onto as often as possible, keeping me on this earth. My heart. My beating heart.
Life is so fucking disgusting sometimes that it physically hurts. Breathing hurts. Crying hurts. Sitting in silence hurts. Waking up hurts. I can't even choke out words. Just sobs.
Even the slight bit of hope I'm supposed to be hanging on to, that somehow tomorrow when i take her into the vet, that they'll cure her suffering in a way that lets me bring her home, breathing.
Am I in denial about her quality of life? Am I hanging on? Is she simply hanging on for me? But her body, her tiny little body, it's betraying her and breaking me.
How does anyone make these choices? How does one accept the guilt of being god, of taking life? Is it humane this way? Is it really? How do you know? Someone fucking tell me what to do.