Today, I’m feeling better. I remembered a few things.
I remembered when I got Mocha. She was a rescue girl, a cattle dog / German shepherd cross, a challenging combination of breeds. When I adopted her, she had a belly full of parasites and she was scared of everything.  
The first week, then six weeks, then six months with Mocha were very challenging. I hired a trainer and we went to weekly classes. She learned to heel like a champ, she learned to walk in the busy streets of Toronto and gradually completely got over her fear of loud noises.
Even though thunderstorms still scared her, she went from trying to claw her way out of the house to coming to me to sit on my foot for reassurance while anxiety shook her body.
Here’s what I remembered today: Mocha actually GOT BETTER. The techniques I learned and used, WORKED with her. And here’s why:
Mocha had the ability to observe her own experiences and modify her own responses over time. In moments of stress, Mocha was able to observe the consequences of her behaviour, and observe whether anything bad *actually* happened.  
Over time, Mocha learned that bad things weren’t going to happen just because there was a loud noise, or she was alone in the house, or that other dog was looking at her.
Mocha was an incredible success in dog rehabilitation, after two years, she was my bomb-proof girl. She became the stoic anchor of calmness in our house.  
With Mocha, we succeeded.  
I am beginning to recognize the overall sum of Happy’s behaviour may have been symptomatic of something congenital.  
Happy had OCD. Everything he did had an obsessive edge of hyper-focus. Fetching, collecting and putting away all the toys in the house, licking, guarding, reacting. 
I always said Happy was the most sensitive dog I ever had. Any new stimulus was instantly overwhelming. We had to desensitize him to nearly everything in his environment, and unlike Mocha, he never reached a decision that things from now on were going to be ok.
Every car ride, new beach walk, new dog or person was a shot of anxious adrenaline to his system. Noises seemed unbearably loud to him, colours overwhelming.  
I felt like I needed a whole specially designed sanctuary for him, where every element of his environment was controlled.
Even then, would he ever relax?
Never once, did Happy relax. I could rub his belly, and he’d look up at me anxiously. He’d cuddle into me and sigh, but go on guard and shark out at Sweetie or other pets if they came near. Even in his kennel he’d growl and shudder at unseen sources of sound outside on the street.
He never progressed beyond his own fear. His negative behaviour was always an outlet for his never-ceasing anxiety. No matter what we tried, how much he was exercised, stimulated, desensitized, trained and positively reinforced, that fear never, ever went away.
I was remembering today all the things Mocha would do when she was relaxed. She might grunt and flop into her bed with a sigh, or roll on her side and paw lazily at a toy. If I scratched her ear or rubbed her belly, she’d sigh and lean into me.
Happy never relaxed.  
The next thing we could have tried would have been medication. I think, we were just out of energy and hope. I just couldn’t risk another incident, at the end of the day.
I was wondering if there was such a thing as dog autism, and I found that there was a similar group of behaviours that sometimes appeared, in a litter of perfectly normal and healthy puppies.
Happy didn’t have all of the symptoms, he didn’t shy from my touch, but would quickly become overwhelmed by the ministrations of other people. What he did have was a compulsive organizing of his toys. Not a problem behaviour, necessarily, but something I had categorized as a convenient quirk. He gathered up toys and put them in his bed. A reasonable, normal behaviour, but one he did obsessively, like he approached everything else on life.
I’m not diagnosing my dog posthumously, I’m just gaining a new perspective, something I couldn’t see for some reason while he was still alive.  
His anxiety was with him from day 1, and unlike Mocha, he was unable to control it.  It controlled him.
Ask a human who struggles with anxiety and you’ll learn its not rational. It’s something that finds many negative behaviour outlets, something that can be managed, but maybe not cured.
Maybe Happy’s anxiety itself was a symptom. I know he found the world to be overwhelming. He reminded me a lot of myself, in kindergarten. He just never found a way past it. Every day was his first day at kindergarten. We did everything we could to show him the way.
Remembering Mocha and how severe her anxiety was helped me realize I wasn’t foolish to take Happy on. I enjoyed animal challenges. I was SO SURE if I did the right things for long enough… Well, you know.
I hope he feels safe now. I’m sure my Mom is taking care of him.