constant forearm licking.

I have a Black Lab Border Collie mix named Porsche. She went through a
window about 2 years ago. (didn't open the window first) A shard of glass
when through her paw between the pads. I got it stitched up and she was
fine, then she started licking the top of her paw, just above the wrist. So
much so that it would be licked raw and bleed.

I went back to the vet, we took x-rays and found nothing. I put the E-collar
on till it healed up all the way and grew fur back. Right when I took it off
(the collar) she went right back to licking. I haven't changed foods, nor
does she have any allergies. I have gone back numerous times. She is still
wearing the e-collar. I don't know what else to do. I thought it was nerve
damage... my vet swears it's not. My vet wants me to put her on Doggie
Prozac. I don't want to do that, but nor do I want her to wear the collar
any more.

TIA for any information.

Jeff