My chow-mix would not let me out of the back door of my home yesterday. What should I do to fix this?

I am at a lose as to where to turn regarding dominance and aggression issues with my dog. As a young puppy, somewhere between 6 mo. to 1 year she was placed in the yard of an auto body shop. She had a doghouse and was fed regularly, but that was about it. My boyfriend visits this shop often and developed a relationship with the dog (I believe she is part Chow & part Keeshond). When I purchased my own home he asked me to take her in. At that time she was 3 - 4 years old. There is also an indication of some sort of abuse as was evident by her reactions to certain stimulus, i.e. fast movements, keys jingling, water, and cowering when men enter the house. She is also very aggressive towards other dogs and acts aggressively when people walk past the house.

Things were not good from the start. She often growled at me and showed her teeth. I worked with her the best I could to develop a relationship - but quite frankly she scared me. Over time she got better, but still on occasion growled, often for what seemed to be no reason. I do, however, realize that she must have perceived some kind of threat After about a year, I hired a trainer to help me work with her. She was not growling any more but showed dominance in other ways, not eating while I was present, not allowing me to put on or remove her collar (again, growling), etc. She even once bit the trainer, not hard, but she bit him non-the-less. For a while my boyfriend fed her and handled the collar. However, overall her behavior improved, in fact she even learned to be affectionate. I did experience occasional growling issues, but much, much, less often, and I grew to love the dog.

We have very little control over Gabby. She must often be lured with treats in order to get compliance (she does know commands but refuses to obey unless given a reward). She can not be taken to the vet because she will not allow them to come near her. Nor can she be groomed by a professional groomer, or us, unless she is muzzled. Muzzling her is an extremely time consuming feat. She is typically isolated when visitors come to my home. We only allow her to be around a select and small group of people.

It is now 2.5 years since I adopted her. For some unknown reason, the behavior has again escalated. Just yesterday she prevented me from going to the back door to let her out. She faced me off and growled. I would like to begin training again, but I'm a little afraid. Also, I am unsure that I can get my boyfriends long term cooperation. (our styles are very different and often, as experienced in the past, has undone any training that I've accomplished). Can this dog be saved? Do I have some real safety issues I need to be concerned with? I would prefer not to give up. I made a commitment to this beautiful beast and feel that she is not to blame. She has been treated poorly by humans in the past and is reacting to past experiences.

Please Help.