Just because a horse has been successful or raced a lot does NOT mean they will never be relaxed, calm riding horses. Far from the truth! In fact, many older runners are well loved on the track because of their laid back nature.
Sadie I totally get what you are saying. Mine is very similar-totally laid back 99% of the time and incredibly responsive, but he does tend to lean into the bit and once he gets rolling there is little chance of stopping him, it's literally like pulling against a wall.
I've found bit wise, less is more. He goes great in a hackamore and best in a neck collar with no rein pressure. If he doesn't have that pressure, there is nothing for him to lean against. How does your mare turn? I would work on bending ALOT, get her really flexible and loose, help break up that "pull against you" because she will have a much harder time bracing when you are only pulling on one side.
Do tons of serpentines, circles, figure 8's. Don't let her get to the point of going faster or being able to run. Lots of transitions. Even if you only trot a couple strides, then back to the walk. The trick is to change things and stop her from getting a chance at going faster or leaning against you, before she even thinks about doing it. Eventually she will become much more responsive and attentive to what you are asking for. Don't forget to praise her when she's doing good!
With her history and lack of proper training, you will have a long road ahead of you retraining all those bad habits out of her. Best of luck, it IS possible, just going to take lots of patience. Don't let anybody discourage you about her being an crazy, untrainable OTTB either
People who think those kinds of things have no idea what they are talking about.