When or How do you know it is time to let it go?

Sorry this is going to be a lengthy post.

I have 2 cats: K is ~14 years old and B is ~13 years old.

I do not know their exact birthdays because they belonged to my ex-bf who is unsure about their ages since he adopted them when they were stray kittens. But he'd been taking care of them for 7-8 years before I took over 'custody' for another 6 years. He didn't abandon them, he just got too busy with work and work demands that he travelled frequently. Then it just became a natural thing that I took care of them thereafter.

K is relatively healthy for her age. She suffered from stomatitis for about 8-9 years. The vet suggested that we removed all teeth but my ex didnt want to remove her front teeth so help K maintain some dignity as a cat (i still do not understand this thinking), so she was on prednisolone for as long as I could remember. When K came under my care, I went "f's sake, let's put her out of her misery and take all the teeth out." Did that, and K's gums went straight from red and inflamed to pink and healthy.

B, on the other hand, led a relatively healthy life. She was constantly terrorised by K since young and developed into a mild-mannered, timid chonky cat which often hid under beds and couches before I turned up in her life. 2.5 years after moving to Shanghai and slamming myself into B's life, my ex and I broke up and I moved out. Knowing very well that my ex actually does not care much about B and has all his attention on K, I took B with me. (K was dumped onto my hands about 1.5 years after I left him)

Last year, in the middle of May, I was cuddling with B when I felt something wet on B’s abdomen and she was in such pain that she almost bit me (she’s not into clawing or biting unless she was in distress). Turned out to be a ruptured abscess which was about 3 inches wide and B needed stitches. Which also meant that she needed to be put under anesthesia. We did a blood test to make sure that her organs was healthy and could tolerate the effects of anesthesia. She was fine. So we stitched her up.

A day after that, she started throwing up, refused to eat, it was evident that her body was shutting down. After throwing up bile a few times, the vet decided that B was not recovering well from the abscess episode. We did blood tests and found out that her kidneys were failing. That was early June 2025.

Fast forward to today, B is at stage 4 CKD. She has Plasma Cell Pododermatitis (Pillow Foot) and there are open ulcers on both hind paw pads. She’s on oral doxycycline and recovering slowly. We have also stopped giving B Lactate Ringers Solution under the skin since 1 week ago because her body was no longer able to absorb those fluids. B has lost all interest in food, even her favourite snacks – I feed her a concoction of wet food, supplements and meds via the syringe 3 times a day.

B is near end of life. I feel like a part of me would die along with her.

So here’s my question, when or how do you know it is time to put B to sleep?