Be prepared a trainer is very expensive. The trainer who had my sister's pup, was exactly what you are explaining in your post. Three trainers came, and it cost her $800 for the last behaviorist. The first two never came back, because he was so nasty. The last trainer took the Lhasa out of my sister's house for three weeks. She left down her guard, and he just about bit her finger off recently, this is a Lhasa.
Everyday you have to remember to put your sheltie through the paces of what some call Tough Love, there is a site all about this. The trainer used chains in a bag thrown at the dog feet, you can also use a shake can but use it as little as you can.
I think toys are great too, if your sheltie brings you a toy, ignore, you go pick out a toy, you and your sheltie play, and when your finished playing you take the toy, and you always win.
If you are eating popcorn, or whatever watching a show, your sheltie receives not one treat with you. Don't permit him on your lap. I know one poster stated also do not permit him in a chair, on the couch. Its called tough love, and you are the leader.
Get him used to moving his bowl while he eats. Throw a treat to distract him and move the bowl once and awhile. I am sure he does not like you messing around with his food. Build confidence you are just moving the bowl of food not taking it away from him.
If he nips, scream OUCH as loud as you can. I heard that helps. NO, is a good one also.
These are just a few things that have worked on fear biters, I can tell you one thing never let up on your guard. Believe me just about everyday my sister has to remind herself to do Tough Love, and she can't spoil him to death, otherwise her dog will take her hand off. Horrible situation, and I feel for you. You got a little stinker on your hands too. I would also call where you picked up your sheltie from, and ask this person some questions, your sheltie had to pull this stunt on her also. ???
Hopefully others can add to the posts that have been posted too.