Good advice. This is how I've trained my dogs. First, you have to train in increments. LIke he said above, you start with small amounts. Eventually, you increase those times so as to build their trust that you'll return. The biggest help you'll find, is wearing that dog out before you guys take off. Not always convenient, especially with this weather. It's slowd down my training of my younger dog, but she's been making great progress. Titan had separation anxiety, as well. Talk about f'n destructive when he was young. Now, I could leave him for 12 hours and not a thing, NOT A THING, is wrong when I come home. I dont move anything or put anything away. IT just takes patience and training.
YOu have to have patience. You can't keep the dog caged in lieu of training. You'll only exasberate the situation. You have to make a decision: Do you have the time/patience and love for the animal to train it properly. If you cannot do that, maybe a dog isn't for you at the moment. However, you're obviously making the effort. There's some good advice if you google it, but I think you'll find much of the same advice you have here. Good luck, man. It's not always easy with this kinda problem. Our new dog has seriously cut into our trips outta the house, but she's turning into a great dog over time.