Hi! My previous dog ( a GSD Mix) suddendly started to react badly at being left and used to bark continuously while we were out, Humphrey however is absolutely fine but after reading threads on here, I changed the departure procedure early on and wonder if that has anything to do with it.
I pop in and out of the house several times a day but make no comment whatsoever, no big "now be a good boy while I'm gone". I'd always made a big deal with it with the old dog and so he did too, probably. When I return, he get a pat and a "Hi Humphrey" followed by a biscuit, once he's calmed down and I try to keep it as low key as possible, that way he sees it as being just normal routine. He sleeps on his own downstairs at night and we do more or less the same at bedtime, - biscuit, quick pat on the head, lights out, that's it and he's no problem.
We're both home all day so it was important to me that he didn't get too used to being the center of things ALL the time, so I do try and not interact with him non stop, I'll disappear upstairs without comment and he just stays where he is.
No idea if any of this applies in your case, but maybe it might help? Other than that, maybe leave a radio on, leave an old t shirt, or something that has your scent on? All worth a try. Best of luck!