I don't really know what to do about it. He'd been acting weird, but he was always a weird dog, I just thought he was being a slightly odder version of himself. Turns out he was dying. I sat next to his body for hours thinking he was just asleep - I even commented to my wife in a video call about how still he was being. I should have realized, but I didn't know he was gone until I reached down to pet him. I don't even know what happened and we're having him cremated so I never will.
Fortunately my wife and kid are out of town so my kid didn't have to see his body and my in-laws helped with all the physical stuff so I didn't have to struggle with that. It feels wrong to have all of these emotions days later but there they are. I keep expecting to see him every time I turn a corner and, while sitting in my recliner, I keep starting to reach over to scratch his head or pet him.
Sorry for your loss. We had the same thing happen a few months ago, and I even posted here as well. I was a wreck for the first week. First dog and had her for her entire life of 10 years. A few months in, and I still get sad. All the things you just did without even noticing are the things that keep making me remember.
I'm sorry for reminding you of your pain.