First off....I am so sorry for you, Monello, and your little jumping buddy, Apollo. Truly.
We had a similar circumstance recently with our 12 year old doggo. She had all the attributes of a happy, healthy, and active dog....aside from the fact that she pee'd all of the time. Thankfully it was always outside. BUT, she would hold her position for peeing, but yet little to no pee would come out most of the time. She would assume the pee position for 5 minutes or so, and you could sense her anxiety about "needing" to pee, but not being able to. Come to find out, she had a cancerous tumor on her bladder. It put pressure on her bladder, leading her to think that he bladder was full - when it really wasn't. She would just stand there trying to evacuate her bladder when it was already empty. The local vet here didn't do much to investigate her condition, but we finally got the bladder cancer diagnosis when we hit up an emergency vet in Ocean City. It was there that they did a sonogram and found a sizeable tumor. We brought her home, and within a few days we scheduled her to be put down. It was a grueling decision, because, like I said, she was otherwise healthy and happy, and she continued to snuggle and give kisses, etc. But her discomfort was painful to her and to us. The morning of her euthanasia, she jumped into the back of the car with a wagging tail and a big old smile. I felt terrible. While I am confident it was the right decision, I am still plagued with guilt over not doing enough, not investing enough $$ into her care. but her age and condition made surgery unrealistic and offered little satisfaction of success.
Best of luck to you, Monello and Apollo.