ewe-neck is just the developement of muscles on the bottom of the neck instead ot the top of it, which is there you want it. It is usually caused by the horse keeping it's neck up with...and almost curving backwards. The best way to fix that would be working on getting her to keep her head and neck down, and on the vertical. Work on collection and so on.
She will definitely not be able to change over night and it may take her a while to keep her head where you want it. When you ride her, ask her to stop, than gently "milk" the rein (small tug...or half halt, on one rein than the other) and then when she puts her head down even the slightest bit, release and praise her.
Just keep doing that and she should slowly start lowering her head and neck farther and farther.
I would not recomment using a martingale or any "training" device especially if she was abused, it could frighten her and cause more damage than good.
She definitely needs more muscling in the hind quarters....some walking/trotting uphill will help with that