Also consider whether your horse is fit enough, or built appropriately to carry the frame you ask. It might be helpful to post a photo of your TB. Lots of TBs lack the muscling on their topline that makes it possible for them to easily carry a rounder frame.
The best solution for this is classical lunging, with a surcingle and sidereins, to getting her working actively and correctly, and to build the muscle she needs to carry a frame correctly under a rider. There's a great thread in this forum on lunging, where Mercedes describes classical lunging in detail, and recommends Reiner Klimke's book on the subject.
Another great advantage for you in this method is that you'll be able to observe her hind end and back muscles and tell instantly when she's working correctly, rather than struggling to feel it while wrestling with her head.
Once she has a good foundation on the lunge line, she will start to seek contact and offer more of a frame under saddle, a sure sign that your work has paid off.
After struggling through reclaiming my first OTTB, I worked all young TBs, off the track or not, this way. Life's just easier if they already understand what you're asking and are fit enough to comply.