In my life, I've had 2 dogs die on me.
The first dog, the childhood dog I'd had since middle school, died when I was 24. She died of old age, so not unexpectedly, but still totally unexpectedly, as all deaths are. Came home from work one day, and she had passed, alone, some time during the day. It tore my guts out. To this day, I feel guilty. And this is gonna sound nutty, but I feel an additional grief that that dog is buried on the property of my old home, where I no longer live...so I feel like I've abandoned her there.
The 2nd dog I got a couple years later. She died a few years ago at age 9. Cancer. The initial prognosis from the vet was that she'd have 6 more months. They told me that chemo would be an option but at that point, it wouldn't prolong her life significantly, and would probably be more pain & suffering than it was worth, so we decided to forgo the chemo. I got 9 more months with that dog, though. At the end, the cancer got to her brain and she started having seizures. That's when I took her to have her put down. I had been looking into finding a vet that could come to my house to put her down, because I thought that would be the most comfortable thing for my dog...but it all just happened too quickly and I wasn't prepared. The last night before her last day, I was at the animal ER till about 2 in the morning...they sent me home and told me they'd call me in the morning with news. When they called me, they told me that I needed to make a decision about prolonging treatment or putting her down that morning. I was prepared to spend another $5,000 in hospital bills, but decided to let her go. The ER doctor told me she thought I was making the right decision, which was consoling to hear. So I drove back to the ER, retrieved my dog, took her McDonald's to get her a last breakfast of an ice cream sundae and a sausage biscuit, and then we drove to the vet's and they put her down.
Both experiences were equally painful...to come home one day and find your dog dead, out of the blue...and then the other scenario of having months to get used to the idea...I don't know which is worse.
Anyway, I feel for you, and am sorry you are going through this. I still get really sad about both my dogs. I hope there is an afterlife and I hope both my dogs are there.