Have you tried to change your behavior? Tusla-Dan didn't recommend it?!? But I will read The Dog listener. It is a great book. Your puppy thinks he is responsible for you, he's not sad because he is alone, he is panicking because he has no idea where you went, if you'll come back and he thinks it's his responsibility to look after you. You need to go back to basics on this.
How about 30 min before you leave you take him out for a small walk and play fetch for a few minutes, a tired puppy is a happy puppy. Come back inside and rest before you put him in a gated off area of your house or directly in his crate. Put his favorite toys, maybe a blanket that smells like you and good chew toys kong etc... Don't make a fuss when leaving a few minutes before. Start putting him in there when you are home, ignore all cries, wait at least 10 min after he stopped crying to go and quietly praise, if he starts crying again start over. Then practice leaving for 5min, 10 min, 20 min. Never leave the same way dogs figure out routines very fast and if you always have the same, he will start becoming anxious at the first sign that says to him your leaving. Never give the dog attention when you come back wait at least 5 minutes (no crying) before calmly acknowledging him. Give him time to calm down.
I don't think this situation is to the point of no return but you need to change your habits now! Your dog needs to consider you the leader in order for him to have a happy stress free life. If you get tensed about the situation leave, go take some fresh air and come back when you are calm.
I know its' frustration and it's sad, but unless you commit yourself to changing, this will continue.
Good luck and please re-read the posts above there are really good advice in them. Keep us updated.
Catherine and Gazou