When do you know "its" Time?

   Sheba is having a really bad day today, and a past few very bad nights. I have never had to deal with the decision of anything like this before so I don't know what to do. Last time we when to the vet, they did blood work and said everything was normal. But everything isn't normal in my eyes when we are at home. She has sever allergy's (or at least that's what the doc called it) so the past 5 years I bought the very best food (allergenic) , she takes all her pills everyday and on time, and we make a visit to the vet monthly like clockwork. She is constantly trying to reach a itch and scratch it, and cry's because she cant, or itching doesn't make it better. Her fur is totally gone on the stomach and making its way to her back. She also has been diagnosed with Canine Cognitive Dysfunction Syndrome. Its kinda like Alzheimer's for pups so that doesn't help with her blindness and loss of hearing. She is getting more and more vocal, like she is confused and scared. She still can walk but has trouble not falling and we definitely don't walk very far.  I pretty much end up carrying her after she goes potty 98% of the time. Every time she eats or even walks by their food bowl I feel like she has a leg spasm or an itch and hits the bowls and food/water goes everywhere. (its OK tho) We have much bigger and heavier bowls in storage that I am wishing we had at the moment sense I have already cleaned the place where they eat maybe 20 times this morning. She is still usually good with going to the bathroom, just not these past few nights. I'll just have to wake myself up when I hear that she is up during the night.
   I don't want her to suffer. I wish she could tell me if she is hurting so I would know what to do.  14 years old, I feel like she has lived a long life. I'm not sure how good it was before we got her but I know the past 5years I have done the best that I could with keeping her healthy. Mikes mother had gotten her from a couple that was getting a divorce. I guess she was around 8 when that happened. I don't know how you could have a dog for 8 years and then when divorced comes, neither one of the people want her. I'm also not quite sure how longs Mike's mother had her, but after she passed away we took her in.

   She is the sweetest dog and our home would not be the same without her. Days like this make me think that it is time to let her go. I am just never confident about it and don't want to make the wrong decision.