How do you know?

I have an 11 1/2 year-old soft coated Wheaton terrier who literally is my everything. I got him when he was six weeks old, in Buffalo, where I am originally from, on my dad‘s birthday, one year after my dad died. He was only six weeks old. He was definitely meant to be with me. No doubt about that. His name is chopper. Chopper has been the only one who has been by my side through every single thing. Through my divorce, being diagnosed with a very scary, chronic, rare disease, being told I have two brain aneurysms, 10 surgeries, more hospitalizations that I can count, losing two jobs, losing most of my friends, declaring bankruptcy, basically I’ve lost everything except for chopper. But I am starting to get worried that we’re getting close to the end. Because he just seems very lethargic, sleeps a lot, doesn’t really eat much besides like treats and human food, which he loves Still and sometimes it sounds like he’s struggling to breathe. Now I have brought him to the vet. He was diagnosed with doggy dementia so he is on a daily medicine for that but they said besides that he looks OK, but I’m just wondering if anyone sort of had a gut feeling, or can give any advice on when they knew their dog was getting close to going over the rainbow bridge. Like I’m bawling writing this. Sometimes I feel like he is looks at me and it’s like mommy I’m old and tired. But there is I guess nothing really wrong with him but my biggest fear is that I’m gonna come home from work one day or something and he will have passed because I know several people that has happened to. I don’t really know what I’m looking for, maybe what I should be looking for I don’t know of a betterunderstanding of. Pictures because he’s the cutest dog in the world lol.