My dog has diabetes and some other complications When will we know when to put her down? :(?

Okay, so my dog is 16-18 years old, no one is for sure... But She has always been in my family, but kind of moved from one clique to the next frequently. My uncle had her as a puppy, then my grandparens, then my mom, and now Dottie (My doggie.<3) Is living with myself and my dad. She has diabetes, and gets insulin shots everyday. But she's getting worse.. It started out with her peeing everywhere, which we will clean up for the remainder of her life no doubt she's worth it. But now, she's blind, and has trouble hearing. She drinks a lot of water everyday, eats good (She's a little chunker.<3) but sometimes, she will pee without even knowing it, and we have to carry her down the stairs, although she gets around pretty well, sometimes she gets confused pretty bad. Also, there will be some days where out of the blue, she will act sick. Like she's just down about something... My question is, is when do we know when to put her down? I don't want to put her down. But out of love and compassion for her, I don't want her to suffer.. And I don't know how to differentiate when she is suffering, and when she is just being her new self, considering the diabetes and other things.. It breaks my heart for her to be in pain.. She breathes very heavily, and snores very loud, she doesn't go up stairs or down very often, but when she does she acts like nothing is wrong. And another thing I think is wrong, is that lately she will pee under the kitchen table, and then lap it up.. And I wonder why, or what is wrong with her to do this.. She also lifts her back legs up, and scoots on her butt... Are any of these things serious? We don't have a lot of money to take care of vet bills, but we make sure she has an insulin shot everyday, and food, and water, and love.. We just don't know when we cross the line of ignoring the signs to keep her here.. Are any of the things i have listed wrong? She kind of has good days and bad days.. Sorry if I'm rambling..