Adopted dog success stories?

My husband, 78, and I, 75, adopted a little seven-year-old Rescue terrier seven weeks ago. He’s the cutest, most loving thing with the both of us at home. But we have discovered since bringing him home that he has terrible separation anxiety. We cannot leave him in a crate, or in a room or even in the house for a minute without terrible heartbreaking crying and barking. We even installed cameras and the barking and crying do not diminish no matter how long we are gone. So we have not left the house together now for over a month. We have spent a small fortune on a behaviorist veterinarian and trainer, and we are doing everything, including putting him on Prozac. We are hoping that sometime over the next several months things will improve, but it’s a challenge. Question, has anyone survived the sort of situation? Also, are we insane adopting a dog at our age? He may be seven but he’s a little so he might have a pretty long life. That part doesn’t worry me as much as the behavior issues. Finally, returning him probably means euthanasia so it’s a horrible, horrible decision whether to keep him or not.