How Do You Know When It's Time? Options

[Bobbie]

My Trevor is slowly dying from a bunch of neurologic problems. Most of the time his pain is controlled by medication given every 3 hours. But then there are the nights that he and I are up for hours. Trevor is pacing, digging into the carpet, shaking his head and whimpering with pain. I give him many extra doses of pain medication, antihistamines and sedatives and then wait for them to "kick in". This can take up to another half-hour. He does not like to be touched at all during these episodes and, since he's almost deaf, he cannot hear soothing words. So I either sit or lie with him on the floor. When he does, finally, fall asleep, I cover him with his blanket and lie down beside him, covering his paw with my hand. That seems to be comforting to Trevor.
Trevor's neurologist says that Trevor will not die of the neuro problems. It will be a quality of life issue. Oh, great.
My problem is this: with each of my other ##er Spaniels, I knew exactly when it was time. This time I have no idea. I do NOT want Trevor to suffer, but there are times when he is doing just that. And yet, afternoons and evenings are decent times for him. And my heart just breaks when I think of him not being here (with me). Then I feel selfish, then I feel scared, etc. etc. etc.

Is there anyone who might be willing to share any advice they have?

I love Trevor with all my heart and soul. He is a 12 year old rescue dog that we've had to almost 2 years. He is such a good boy, even with all his "issues" and I want only the VERY BEST for him.
Thanks so much!
Trevor's very sad mom