This sounds alot like vestibular disease. Vestibular disease is a disease of the inner ear. It might cause stroke-like symptoms in the dog and will cause him to walk in circles, drool, favor one side or have one sided weakness.
There ARE treatments for vestibular disease if you suspect this. Long term antibiotics are usually used after an exam confirms this diagnosis.
The symptoms can be reversible if you catch it early enough so you will want to go ahead and see the vet.
As far as quality of life and longevity are concnered, this is a pretty tough question to answer and I really think that it's a personal decision that you must make.
I counsel people on this topic all the time, though, and I can tell you what I would tell them.
You should choose three things that your pet loves to do. It might be riding in the car, eating cheeseburgers, going for walks, chasing a ball. As he gets older or, maybe, sicker, he will cease being able to do these things that he loves. When one thing is gone, like chasing the ball, it's ok because he might take up a new activity like sleeping in front of the fireplace. However, when you can no longer find even one thing that he loves to do, it might be time to reevaluate the situation.
As far as guilty feelings go, you should not feel guilty if you decide that your pet is no longer living a happy, content, productive life and you help him to go on to a better place. You have to remember all the good times that you have had with him. We are lucky in that we are allowed to help our pets to move on. Sometimes I wish that we could do the same for people.
Let me know if you have more questions.
I am very sorry about what you are doing through and truly feel for you.
