Argh I typed a reply and I lost it

, so I'll start again. Sorry for the ramble

You sound like you have been through alot. Separation anxiety is very common with rescue dogs. The usual treatment is working on coming and leaving with the dog - desensitizing, ensuring it has had heaps of exercise before you leave and doggie daycare while you work at resolving the issue, BUT you sound like you were dealing with VERY severe anxiety seperation which usually means a combination of meds a behaviourist, and lots and lots and lots of time, and like you said when you are working full time it is very hard to do - don't beat yourself up about it, because you did the right thing in acknowledging it was something you could not deal with, it just wouldn't have been fair to the dog to have kept it and not be able work on it. Here is a story on a
Springer Spaniel who sounds just as you are describing (just so you can see you are not alone

). As for not ever being able to be rehabilitated I think with the right owner who has a heap of time and the money to invest in the dog and getting a great behaviourist it could be. Sorry I couldn't be more helpful.
This link has some further info on
Separation Anxiety I did have another one, found it, this one is from the
Dog Trainers Network