I'm also sending lots of good thoughts & prayers that Mabel's feeling better soon. Two quick thoughts though in the meantime:
1. I also didn't know if I'd know when it was time, but somehow I did. It was one of the hardest, most heartbreaking decisions that I've ever had to make, but I did know that it was the right thing for her.
2. On a happier note, while Halley was ill and sometimes had no appetite, I would make up little "vienna sausage" shaped bits of her dog food and just pop them down her throat. It's much easier than it sounds if Mabel is as sweet and laid back as it sounds from here. It was very gentle and Halley didn't seem to mind at all. Plus I'd pet and snuggle her while I did it, so that helped, too.
I fed her this way 2 or 3x a day when she wasn't eating. We used a canned dog food that was ground up pretty smooth, but fairly dry, so that it wasn't too mushy and it worked really well. I was surprised by how much she'd perk up after I fed her, it seems like their blood sugar must drop pretty fast when they're hungry, but they feel too bad to eat, so the problem just snowballs. If you'd like more info, feel free to PM me. I should mention that though it was my idea to try this, it was vet approved.