You've gotten some great advice from others above. I would add that often we assume that dogs understand things when they really don't. I did a test a few weeks ago with my dog and was quite astounded to learn that cues I thought he knew he really didn't. I do regular weekly training classes with him and he is five. He understands all of the commands but where we get crossed up is that we humans give so many unconscious cues that dogs interpret as the actual cue.
Sounds confusing but for example if I stand totally still and make no eye contact with my dog and give the command he did not comply with all my commands. If I gave that same command but join it with a hand/body signal or just the way I glance then he is quick to comply. That taught me that sometimes our cue is not what the dog thinks it is. When you add in that your dog is quite submissive it is quite possibly that she just gets overwhelmed and shuts down when she doesn't understand.
LP's book and video suggestions are excellent. I personally am not a fan of the "end of my leash" show. I think Brad's techniques can be useful for some dogs (confident, pushy) but a dog like the one that you describe will do soooooo much better with positive training methods (treats, clicker, praise, play etc). You need to work so that your dog thinks you are the best thing ever and has great trust and respect for you. In return you need to be sure to never push your dog beyond where she is comfortable and listen to her body language to know when she is overwhelmed.
"Never doubt that a small, group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has." - Margaret Mead