I've Cried All Afternoon

Right now there is no Taxdog. Mo had to be returned to the shelter. In the last post, I mentioned how he was aggressive. It seemed to be a mix of protective and fearful. But in the last few days the aggression has gotten worse. He has directed a little to me (I tried to move him off the bed) and he has strained to get at anything I talked to. This happened at home or the office. He would get very worked up if someone tried to lean over the office desk to see him.
Today Mo was supposed to get all his shots and a heartworm test. Two weeks ago, he was fine at the vet's office. A little barky at first but calmed down and was greeting the other dogs and people on the way out. He was fine with Cindy and Carol. Today, he wasn't bad until a man came into the waiting room and he started barking, growling and straining on his leash. In the exam room, he was barky at the vet tech but was fine with one of the office cats. That is until I tried to pet the cat and Mo lunged at it and me. Jealousy or protection-I don't know. The behavior continued to get worse when Cindy came in to examine him. He growled and cowered. We discussed his behaviour and what could happen, especially at the office. And we talked about returning him to the shelter.
I still was not convinced when I left the office (no shots or tests - he was too worked up). The more I thought about it the more I realized that I didn't have any option but to take him back. Right now I don't have a lot of people come in but it would take him 3-4 minutes to calm down and stop barking when someone came in or the door blew open. So when I get busy, he would have to stay home for over 12 hours a day. That is not good for a people dog. Exercise is made hard by him wanting to attack everyone he sees on a walk and my yard is not big enough for a lot of running or walking. Gizmo has also not shown a lot of interest in toys or treats. The treat problem (he doesn't like or hides most and he's not good at taking them from my hand) was another big factor. How can I train a dog when I can't use treats as the first reward? I am sure there are trainers out there that might be able to train him, but I can't afford that. I also can't afford a lawsuit if he were to bite someone.
If he had been getting better as he and I bonded, I would have held out hope. But as we became closer, his aggression to others increased and I don't see how that will change with the resources I have available.
Gizmo was in my life less than 3 weeks yet I can't stop crying today. I am sure part of that is guilt. But when he was good, he was a great dog. But when he was not good, he was a dangerous dog. Hopefully, he will find a home soon.