I agree with every word Nancy says.....our Harry was exactly the same.
I'm used to being with him 24/7 - but there are times you just can't take them, for instance when it's boiling outside and you have a loooong hospital appointment.
It does get easier as they get older and more secure - once they realise that you WILL come back!
The pup-sitters that I've used to pop in for the odd hour have said that he's become much more relaxed as he's got older, he'll watch and wait now instead of crying and getting into a trembly mess!
I really wish you luck, and please try not to be too worried - it WILL pass
Sally + Harry x