You will know when it is time
I promise you will know. Do some serious soul searching about the quaility of her life and also about the discomfort you and your family are experiencing while trying to figure this out. There will be a moment when you decide that enough is enough for your pup. When an animal is "put to sleep" that is exactly what happens. They fall off to sleep. No more pain and upset. We are all going to go someday and you, my dear, have the opportunity to end whatever pain this pup is in, when it is time to do so. It is such a powerful time, but also a huge responsibility - very difficult, at best.
Decide what "quality of life" means to you, doggie wise. How much do you want her to tolerate?
The process is painless for the pet. The emotional part is not painless for the humans.
If one can spare a pet from struggle, one should.
Good luck. It is a tough call. You owe it to your widdle puppy, that 14 y.o. border collie friend and love of your life.