When to say goodbye

Warning, not a happy post. This past September our pet beagle Sweetpea suffered a fall down our front steps and slipped a disc which led her to be paralyzed in her back half. After a very expensive stay in the Vet hospital she was able to come home and do rehab and get some of her mobility back. She spent months in her crate and now can walk, not well, and can never do stairs again. Which is tough because we live in a raised ranch. So she must be carried upstairs (which is painful for her) or she stays in her crate. On top of that she has lost her sight. This breaks my heart because she is a stray that we adopted who was horribly horribly abused. She is not a typical dog, she doesn't like toys, balls, or even really seeks out attention. She does love to be pet and snuggled but only if you go over to her. She has to be on what I refer to as puppy Prozac just to keep her stable and not having stress diarrhea. We love her dearly but we have come to the conclusion that she is a burden that is too heavy to handle with two kids, and more than that her quality of life is not good. My husband is more ok with this then I am because he is the one the mainly takes care of her. I am having a harder time mainly because she is such a generally sweet dog who has had a horrible life and I wish there was more we could do. Also, two years ago this October we put our other dog down (that dog was my world and it crushed me to put him down) and I feel that Sweetpea is our last connection to that dog. I can't imagine living in a house without a dog but I am thinking that the only reason we haven't done this sooner is because we are selfish and not thinking of what is best for her, then I question of the reverse is true, are we putting her down because it would make our lives easier. Then this AM when I took her out to go potty I bent down and pet her and looked into her eyes and saw what I did in my other dogs eyes which was sadness and that it was time. Now, the only unanswered thing in my mind is what do I tell my son about his dog. I can't even begin to think about that now.