[Help]My dog still attacks me over severe possessiveness. Please help, I don't want to lose him.

First and foremost I am making inquiries for professionals in my area. And I'm a bit a emotional right now so I might forget some pertinent details. And sorry for the wall of text.1 year ago I adopted a dachsund/rat terrier from a rescue shelter (who is now 2) and was told he's "skittish." That's when the fun started. I was bitten the first day when I approached him trying to hoard/bury a treat. As months went by I did what I could making him "work" for rewards and scolding the hoarding. For the majority of this time the incidents where he attacks me are over animal part treats or food he's not supposed to have in the first place. The only animal part I have successfully taught him to give up willingly on command and calmly is a bully stick, so I give him those to chew on exclusively. Today a pizza box fell off the counter due to the wind, and walking into the room is all that prompted him to growl deeply before turning around and attacking me. For 12 pounds he has a powerful bite enough that I have to be cautious where I put my hands and hope I'm wearing shoes. In these episodes he has a unique growl where it's almost a mix of crying scared and fight or flight. After he lost interest in what was left in the pizza box he pooped on a pad I leave by the door (he's trained to both go outside or use pads when he really has to) and it was soft probably due to stress. He then acted as if nothing happened. I cleaned up the mess and I've been ignoring him, gating off an area of the house he's in.This isn't the only time he bites but it's the only time he initiates aggression. Socializing him with other people and dogs had been it's own nightmare and he's also terrified of harnesses whether they are step ins or over the head and will bite out of fear if i try to gently but forcibly put one on him.I adopted him alongside a Chihuahua who is one year his senior. The foster dad said "they spent plenty of time together" but the Chihuahua does not like him, used to bully him over treats and food, but I nipped that in the bud. The Chihuahua is submissive to people and my commands, just not other dogs. Sometimes they fight in the middle of the night if they touch each other and the Chihuahua freaks out about personal space but other than that, they spend their days lounging right next to each other. He's kind of apathetic about the first dog's presence and has no interest in being friends.Realistically I want to know if he's too far gone. I accepted a long time ago I might not ever have a "normal" dog but now I'm scared to admit I nor any family I have present or future will be safe around him and it's time to give him up. Any advice is appreciated. via /r/dogs http://ift.tt/2ryn65O