I'm sorry. I'm going through the same thing. It's confusing and distressing. I have put a dog to sleep before, but it was obvious in the past.
My dog is over 15, medium-sized, and a mixed breed. She has also been going downhill for a while. She is old. I think most of her symptoms are due to arthritis. Her mobility is progressively less. She's losing muscle and looks skinny, but still mostly eating adequately.
At times, our dog will wander around the house and bark, sometimes repeatedly late at night. I'll try letting her out or getting water or treats. Sometimes it's ok, and sometimes she doesn't want any of that and keeps barking minutes later. Some nights I'll stay on the couch so she doesn't wake everyone else in the house up. Other times, she's fine for days. At those times, I think she'll be around for months.
Our dog has always been stoic. It's hard for me to know if she's in pain. She's never cried or whined. As an example, about 10 yrs ago my husband and I would both take the dog for runs/walks at different times. She started being less than excited when we would get her leash. I talked to the vet at a visit. He examined her and said she seemed fine but should probably have x-rays. He said she had a fractured front leg and was probably in a lot of pain.
I did recently schedule a visit for end-of-life discussion. The vet talked about our options and how it would work. She said our dog seems weak, but not necessarily in pain. I know a little more what I'm in for now. It helped. Our dog takes Novox and Dasuquin and now gabapentin.
We took her to our friends' house last weekend. They have fostered several old and sick dogs. They told us we would know when it's time. Similar to BarelyBreathing, they said her eyes are clear and bright and that's good.
You've been through so much these past few years. I'm sorry. We just have to trust it will all work out I guess.