Aggressive lab puppy?

Question
My 7 month old labrador retriever puppy Estelle is a "hard puppy". She has a
very strong prey drive, and has hyperactivity disorder. She gets three 1-2 mile
walks a day, and I take her to the dog park several times a week. She
excessively barks, and it's not an issue of lack of toys or attention. Estelle
also has a very hard time with the word "no". For example, she will rudely
jump up while I am sitting on the couch, and start nibbling on clothing. When
given a very firm "no", she will have a major meltdown, (barking, lunging,
biting), even when a proper chew toy is substituted. I have tried numerous
methods to stop this behavior, including: walking away, lifting her front legs
off the ground and holding her in between my legs until she calms down, and
rolling her over on her belly and firmly telling her NO. As soon as I come back
into the room, or release her, she starts barking, biting, and lunging again.
She's been this way since she was 2 months old, although when she was very
small she was not able to cause as much damage as she is able to now. She is
nearly full grown now, and the behavior is only getting worse. She doesn't
have food aggression or aggression towards other dogs or people. I have
consulted a professional dog trainer/behavior specialist, and he diagnosed
her with hyperactivity disorder. Estelle has been to puppy socialization, basic
manners, and basic obedience, and has passed w/flying colors. She is
extremely intelligent. She was promoted to advanced obedience, but upon
throwing a tantrum in the middle of the first class, she was kicked out.  Just
the smallest thing can set her off, but the main trigger is when she doesn't
get her way. My gut tells me that this is not normal puppy behavior, and the
behaviorist/dog trainer agrees. She was spayed two weeks ago, and I've tried
a lower protein diet, as a possible solution to the problem. Estelle had a
major meltdown a few days ago, and seriously bit my mother multiple times-
- not puppy nipping, but actual biting before she could be brought under
control. My mother was walking her when this happened. Estelle wanted to
go in water, but was not supposed to due to surgery. When my mother told
her no and tried to redirect her, she snapped. And now I face the difficult
decision of where to go from here. She doesn't get left home alone, (no
prolonged time in a crate). Although Estelle has her good, sweet times, I have   
given up on the prospect of  her becoming a therapy dog, one of the reasons
I bought her in the first place. I really don't know where to go from here.
Estelle is going in for a lyme disease test, the only thing my vet thought
could be a medical reason for this behavior.  I'm beginning to think that she
isn't the right fit for me. Is Estelle a lost cause?

Answer
Some Labs are difficult.  Spaying was a good step, but not a cure all, and takes time for the hormones to work their way out of the system.  Talking to the vet was a good idea too, even if it doesn't help.  

A year ago, we were struggling with a difficult Lab we were raising for a dog guide school.  They need independent, strong willed dogs, but she may be too much.  She did make it through her training, but hasn't been matched with a partner.  I think they are waiting for somebody that can handle her.  Even those highly trained, highly experienced pros can't make a reliable dog guide out of every dog.  

Walking away may be the best way to stop Estelle's biting.  Loss of your company can be a powerful tool.  You will have to do it time after time, and each and every time she bites even a little.  

Many people, maybe including your trainer, have never dealt with a truely difficult dog.  I see their posts all the time on forums thinking what worked on their easier dogs, will work on all dogs.  It is vital that she accepts you as the leader.  There is some stuff at  www.dogbreedinfo.com./topdogrules.htm that should help.  

Give her some more time.  She may settle down.  I hate to suggest rehoming any dog, but if in the end you must, look at some of the security work.  She might be just what the DEA needs as a drug sniffing dog.