My dog's separation anxiety is only getting worse

Hi. I'm going to try to keep this as short as I can, but please bear with me. I have a little westie, he's 12, and I've had him since he was a pup. He's well-trained, but the problem is that his anxiety is getting worse and worse by day. He defecates and destroys when I leave the house (even if just for 10 minutes), and goes absolutely nuts (barking, running in a circle, attacking the first shoe/toy he can get his mouth on shaking it until he chips a tooth/breaks the object) whenever anybody shuts any doors inside the house. This includes bedrooms, bathrooms, etc. This behaviour began when he was approximately two years old, and has only been progressively getting worse ever since. I've tried MANY different trainers (which did nothing), medications (which only made him drowsy but didn't help with the anxiety responses), pee pads (which he always destroyed), diapers (which he would take off), huge kennels (which he constantly urinated/defecated in), a citronella collar (which was effective for a couple of days, but to which he became habituated to, rendering it completely ineffective). I'm at the end of my rope, I don't know what to do anymore. My roommates have been extremely patient with him, but I know I can't let this happen forever. A veterinarian suggested about 5 years ago that I put him down. The dilemma is, though, that I love him to death, and he's a great, normal dog when I'm home (and doors aren't being closed). He'll be great, affectionate, and fun up until I need to use the washroom. He'll cuddle on me until someone closes the door to their bedroom, at which point he loses his mind, scratches me up all over, and runs off in a panic. It's draining and stressful. I don't know who I am to chose whether or not he gets to live or not.. it's quite the ethical dilemma. What do I do? Again, everything has been tried for his anxiety, and nothing has worked.

There's a good article (online) on "dealing with dog separation anxiety" by Cesar Millan.

Well it would probably be great to begin with if we didn't keep animals as pets and interfere with wildlife, but that's besides the point. I liked when I had pets in the past, I get it. I think that keeping a dog in a place you share with roommates is difficult. I'd almost suggest changing your living situation for the sake of the dog. However, as you stated, this dog is 12 years old - he is getting up there. I wouldn't put the dog down unless it has organs starting to shut down and it's in pain. Imagine if we put humans down whenever they have quirks? Like everyone would be dead. I'd just deal with this for the remainder of the dog's life. If your living situation happens to be changing soon anyway, then great! Give it a quiet room with nothing it can destroy whenever you are away. For now, Idk, maybe try to give it a space of its own if you can offer it one. Some corner of the living room, perhaps, where he has plenty of space, a bed, toys, and no furniture or cords to mess up.