Awww, how lucky for your girl that she has found you. I certainly feel for you. I don't think Meg has ever been beaten, but she is naturally a very 'soft' dog, and does not handle any correction well; even a verbal "Uh-uh" will have her slinking out of the room or under a table. If you do a search for my name under the training section, there are at least two posts where I got some great suggestions on building her confidence.
Have you tried a clicker yet? I know some dogs don't like the noise, but if your girl is okay with it, the clicker could be a godsend. Paired with really really good treats, it makes training a positive experience and allows your dog to earn her own rewards. I love it because I don't have to use any corrections when I'm working with the clicker; I simply hold off until there is something (anything!) I can click and reward.
When Meg is having a particularly sensitive day, I have to be very careful that she doesn't constantly get things wrong. If we are working on the tire jump (I do agility with her), and she goes under it and doesn't get a reward twice, she says, "Forget it. I'm an idiot, I never get anything right. I'm done." So, if she misses something more than once, I automatically ask for something simple (lie down, over a jump, or even sit) that I can reward and praise for. It keeps her interested in the game.
Do you train with treats? If your dog will take treats from you, and she has "sit" down, I would just spend a lot of time doing that for now. Let her get a lot of things right for a while and see if it helps her confidence.
Good luck with her, and be sure to post updates. I know how frustrating it can be to work through this, but it is such an amazing feeling when they start to come around.