Hi, I have a 5 month old tibetan terrier and am trying to deal with her huge separation problem. After leaving her for only 5-10mins at a time with a filled, frozen kong , we thought we were making progress. We would ignore her going out and on return.  On listening to the tape recording on our return she was howling and barking but not non stop and on two occasions we came home to her sitting quietyly on top of a chair looking out of the window. Then suddenly she stopped ignoring us on departure,ignored the treats and was at the door jumping up and howling at us - couldn't even get her to sit, she was totally unfocused and upset. My trainer at puppy school has advised going back to closing doors in the house, putting coat on without going out etc. I discovered on doing this that housekeys seem to be the trigger, so spent the next day jangling them and she seems not to be noticing them now.  At the moment, I feel we are never going to make any progress with this and it would be so lovely to hear of actual success stories and the methods used. She won't be confined in a crate so that's a no no. She will go in of her own accord to have a snooze or when we put treats and food in, but close the door and she goes ballistic.

i had the same problem with a boxer who was used to being with another boxer. we had to do the same as you but we would only leave him long enough to walk out the door and be back in under a minute,  i think by walking out then nearly straight back in it helped a lot. we would do this a few times a day and then increase to walking down the road and back. five/ten mins is a long time to a dog when their stressed so very small amounts of time on a regular basis is best.
Also make sure you dog has had a good walk before being left, lots of running around and games to tire the mind out as well as the body. you can do training exercises which will really tire her out ie practise your commands you've learnt etc.
ignore the dog for a good while before leaving not just a few mins and also give less attention when your at home for a few days, only giving attention when you decide not the dog.
we managed to crack this in a couple of weeks but it can be a slow process. keep going i'm sure your get there soon.    

Agree with everything Bear has said. It seems something has happened while you weren't there to set her back so I would also add try leaving a radio on - it's amazing how quiet a house is when you're home alone and noises that wouldn't normally upset her may well have done when she was left. And you might want to try rescue remedy in her water for a few weeks to help her cope with her stress.

Good luck - I'm sure she'll settle.

A bitch that I fostered who had spent months in kennels once attached to her new owner had terrible SA, it took 6 months, but she is fine now.

Routine is key for these dogs to give them confidence that you will return, as well as the usual tactics your using.

Thanks for all you comments some good advice given - Bear I was very encouraged to hear you succeeded with your dog, gives me some hope.  I will start just going out of the door and coming back in today.  Have been making sure she has been exercised   and has had her dinner before leaving her so she's tired. 
Stargazy - I am going to get some rescue remedy today, I have a DAP collar on her but it's only been on a few days and hasn't made any difference yet.  Yes I do leave the tv on for her and have also just got some music for dogs to try as well.