I agree that I would retrain the down from scratch to build the dogs confidence. However, to make it more difficult for the dog to "roll over" when she is downed, you could place her in between two obstacles such as boards or whatever else you can think of where there is no room except to down. Immediately when she downs reward her and praise her.

In competition obedience using the obstacles as I've mentioned above are sometimes used for teaching straight sits and downs. I'm sure it would work for you too.