Oh, man. Liquide, you have my sympathies. I had to say goodbye to cats that were with me for 15 and 18 years. It's damned hard.
One thing to know is that nobody can tell you how you should act. People get very attached to their pets, and everyone deals with grieving differently.
What you face now is rough, but there's something you need to know in order to deal with it properly - there's a common image that an animal that's getting old will eventually quietly and peacefully slip away in it's sleep one night. That's rare enough to be nigh a myth. Generally, an animal that's building up the many ailments of age will linger a long time, and his passing naturally would be far, far from comfortable.
You've given Linus love, shelter, food, companionship, years of friendship. There's a point, though, where trying to continue to give him those things won't be a gift anymore. He'll be looking to you to give him the one last gift. He cannot give himself a painless passing, Liquide.
Only you can say when he's reached that point. He'll need you to watch, to see when it is that he's not enjoying life anymore. When he's got too much pain, no more will to eat, or to move about. Then he'll need you most of all.
Getting
yourself ready to handle this isn't a small thing. You can have a day or two for just you and your pet, where you get his favorite toys and foods. Get some last photos, or make a scrapbook. I the US they sell these little clay plaques, so you can get a permanent paw-print impression. Do whatever feels right to have something to memorialize him.
And picking up afterwards isn't easy. If you want to be there when he goes, be there. But if you feel it'd hurt too much, don't. He'd understand, if he could. When he's gone, have a funeral, if you like.
I'm friends with more than my fair share of veterinarians, and have some other resources at my disposal in dealing with such things, if you need them.
Peace unto you...
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