I've been having trouble getting my 10 year old OTTB mares head down on the bit, in the bridle any tips on how to get her head down or to train her to get her head down?

Draw Reins worked great for me
Your main issue is that you are searching for the wrong goal. What you need is for your horse to engage their hind end, lift their spine and use their topline. Once this happens, the horse naturally bends at the pole, and curves their neck.
One of the biggest issues with horse back problems is that people skip bringing the horse together, and try to wiggle the horses head down. This causes their back to hollow even more, and can result in detrimental back issues such as kissing spine.
You need to first seek out consistent rhythm, then suppleness, contact, straightness, then impulsion (and later collection). After establishing a rhythm, What you need to do is get your horse moving forward with lots of impulsion, and bending, pushing from your inside leg to outside rein, with even pressure on your reins. You should not have to pull on your reins, but create a barrier so that all the energy coming from their hind end is sent down a funnel and has a wall (your hands) to stop at.
The biggest tool to help you right now would be half halts.
Id reccomemd getting a coach to help you with getting your horse ahead of your leg, pushing from the hind end and coming round.
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