I'm sat here at work reading this thread with tears in my eyes!
I really, really hope your dog makes a full recovery. I've had a dog my entire life and loved every one like family.
We've only had 3 dogs in about 38 years. Our first, Bess (a border-collie cross) lived to 17, then Chloe lived (a bit of a 'Heinz 57' mongrel - a bit of whippet, collie, etc.) for 16, and our current dog, Lottie (a blue merle Sheltie/Black labrador cross) is 11.
It's horrible to find that your pet's ill. Lottie's been diagnosed with mitral valve insufficiency - basically one of her heart valves is weak. The hot weather back in August caused her to have several attacks, which are terrifying to see happen. She's on a regimen of tablets to reduce the fluid build-up caused by her heart condition. Although she's not the dog she was this time last year (she's so, so skinny now - and she'd always been quite solid), she's still happy - enjoying her walks, rolling about on the carpet and belting off down the garden to chase squirrels. Of course, all those activities are interspersed with some nasty coughing fits, but she only has one of her attacks months apart now.
I think the important thing is that you know your dog. You know when they're coping, when they're happy... and when it's too much for them. Like another poster said, animals do tend to hide their illnesses, so look for key signs like them being off their food. Lottie's not quite as much of a pig as she once was, and can't finish her usual-sized meals, but she still asks for her dinner on-time, and tries to get away with begging for our food
Lottie's had 7 months since she first became ill. We have good days and bad days, but the good ones outnumber the bad. I hope I'll be able to know when 'the time' is, but it's not today
Here's a photo album of Chloe and Lottie, and (so they don't feel left out!) our
house rabbits!