Broken Neck-Line?

My horse was trained to ride with no it and bo bridle, not even a hacamore, so they werent taught the correct head carriage. So now I am starting dressage and I have to have a bit in her mouth (obviously). I did have an arabian trainer, but that was for regular showing. So now she has a broken neck-line and it is very undesirable... does anyone know a way to get her neck to round out? I am aware that it might take a few months. I am ready for it. I only use a double jointed snaffle, a KK Ultra loose