Senior Dog

Question
This is a bit of a tough question that i have been mulling over for a few weeks. How do you know when your dog has had enough? My poor dog. A 13 year old Lab/Spaniel cross has lived with us for the last 3 or so years. (since her owner passed away) Initially she attached right onto me. Sleeping in my bed, following me around, begging for treats etc. We joke that we should have renamed her shadow. When I shower or use the bathroom she is either in the room or just outside the door. This past year has not been good for her. She has had re-occuring ear infections that take forever to heal. (with regular cleaning and drops from the vet). In the spring she started licking and biting herself so much, her hair was getting so thin. The "soft" hair was falling out and only the brittle hair remained- main areas were her back, legs, and by her tail. We tried everything. We were told she needed more protein. So we changed her food. (we had initially kept her on the brand that came with her/a cheaper brand) We changed her treats and added this oil stuff that was recomended for her. None of that helped. So we went back...changed her food AGAIN and were told she probably has allergies. They put her on prednisone and an antibiotic- which incidentily helped with the ear infection that month too...The meds ended and her hair appears to be in good shape. Her licking is all but stopped again. The vet suggested she was allergic to fleas but we have never seen a flea on her. So just when i think my girl is on the road to recovery from all of this I am home from work one day and she will not get up from her bed. She laid there almost all day no matter what i did to try and coax her out of it. I was very concerned, she is usually a very active dog (does runs and jumps with my daughter regularly in the summer) She struggled to get up and just stood in front of the couch when she wanted up, she couldn't do it. She couldn't go up and down the stairs to go pee outside ( i had to carry her) No small feat at her 40-50lb weight. Anyways. I took her to the vet that night. **She went nuts** a little spaz at the vets office. The poor vet couldnt' get an accurate reading off of her because she is so excitable in the vets office. I am sure they thought i was dreaming it all. I laid on the floor with my dog that day and petted her, listening to her breathing heavy and snoring as she now does in her old age, worried that when i got to the vet they were going to tell me something terrible. She couldn't walk barely, her back legs were not working for her. They did blood tests (after giving her a tranquilizer to calm her) and found nothing. She was given antinflamatory drugs that appeared to settle the pain and give her some movement. However she slept for days, waking only to eat and use the yard. Since then (about 2months ago) we have been carrying her up and down the stairs on bad days when she appears to be struggling. Sometimes she waits to be picked up, other times she runs up like a pup. She struggles to put one back leg up then the next to get up on the couch. Sometimes easier than others. When walking her back end just gives out at random times...she falls flat. Sometimes she gets off the couch and does a faceplant. Her ear is bugging her again. Regular cleaning has not helped. Now she rolls on the carpet like her back is itcy, whining and wiggling around...dragging the side of her head on the carpet.
This last couple months she has stopped coming into the bed with me. She doesn't even come into the bedroom. (there are a couple stairs to go down to get there) Before she would whine to get there. I have lost my shadow. She sleeps long days and nights. When we come home...quite often she doesn't even notice anymore that we are home..a big change from the dog who bowled me over 6 months ago.
I see my options as this:
1. Do the best we can to keep her comfortable. 2.Go to the vet and do the tests they have told me about (ones to see if she has a neurological problem or spinal problem etc etc) the tests are very expensive and honestly i fully believe that putting her through these tests and possible operation that they are thinking would be more harmful for her than anything. If i take her to the vet she will be excitable and it is difficult for them to get any handle on her true condition.
3. Make a judgement based on good days verses bad days and decide that it is her time to be "assisted to sleep."

How does one decide what to do for a senior dog. What is in the best interest of my dog.
I know there is tons of information here but i am just trying to give you a full view of my dog. If you need more information. Just ask i'll try to answer as best i can.
Thank you so much.  

Answer
Hey Karla. This is a very rough thing to have to go through & I feel your pain. I am a FIRM believer in the "quality of life" aspect & not being selfish. We kept one of our own dogs alive longer than we should have & it opened my eyes. Quality of life can be described as: the ability to eat & drink on own, ability to go to the bathroom on it's own, being able to move around comfortably, able to maintain a healthy weight, etc. I know this might sound crazy, but you can look in an animal's eyes & see when they're ready for you to let them go. Being how old she is, I don't know that I would want to put her through everything that goes along with a neurologic exam. This can become very pricey & very invasive. You need to weigh the good & the bad days. You want her time on this earth to be as happy & as pain free as possible. There's no easy decision to make here, but you're her owner & you see what she has to go through everyday. It's honestly all up to you. Just know that we as people have a wonderful gift that we can give to an animal through "assisting them to sleep." We can easily take away any pain that might be causing them to suffer. I think you are a wonderful person for taking her in & being such a great dog owner. You see her everyday & what she has to go through. That should be able to help you make your decision. Please let me know if you need ANYTHING at all. Take care!
-Heather