Could you please help me feel better?

My just turned 5 year old, small, yellow lab/JRT bit me Tuesday. We got him from the pound when he was 6mos. For the past 9mos though, he's been an inside dog. He was super trained and mild mannered. He was my 6th dog. He was my first small dog, and the first dog I had ever gotten from a pound/shelter. The second time he tried to bite me I was laying by the patio door in the living room, I was an had been rubbing him for the past 30min. I made No sudden movements but he turned toward me and went after me twice. He missed the first time cuz I jumped, I was laying on my stomach. I rolled with my hands over my face trying to get up and he did it again. This time I have a tooth scratch and bruises on my head. My mom threw him in his kennel cuz he was acting crazy. She didn't want him inside in the first place. The first time he tried to bite me was a month ago. We got him his first rawhide bone and he guarded it fiercely. I crawled past him looking for an earring on the floor and he tried to bite my face. I didn't understand because he has never been aggressive with us with his food or any of his toys. We have to get him put down Saturday. I feel terrible, my mom wanted to have him killed. She was compassionate torward me, but hates the dog with a passion. He is staying outside for now. :( We talked to his vet, and they said the same thing.