Very typical of most OTTBs, she is tense through her back, hollow and not "through". She also has not engaged her hind end and doesn't come close to tracking up. I would focus more on relaxation, getting her to stretch down, and working long and low before even thinking about "on the aids", a frame or an outline.
Dropping your hand down below the standard straight line between elbow and bit may work short term to get the horse to drop its head, but it creates more trouble long term in terms of correct training. It's also incredibly punitive, creating direct downward pressure on the bars of the horses mouth.
Dressage is difficult for TBs is general because they're not bred or built for it (Yes, I know there have been fabulous exceptions); particularly hard for OTTBs because they've had traveling inverted and bracing on the bit confirmed by all their previous training.
Your mare is attractive and seems willing and like she's being a good sport about what she's being asked, which is a huge plus. It's hard to evaluate her athleticism from these photos, but certainly she'd be capable of doing Intro, Training and First Level.
However, I will tell that reschooling an OTTB as a dressage horse is a long, difficult task, sort of like emptying the ocean with a teaspoon. Relaxing the back and stretching is the key. It can be done, but one day after months and months of work, you'll be happy your OTTB stayed on the aids for her whole Training Level test; and you'll realize that riders on warmbloods achieved that milestone in 1/4 the time or less, which much less sweat than you did.
Disclaimer: I dabbled in dressage, it was not my primary discipline. If anabel, Spyder or Mercedes post in this thread and say something different, please pay attention to *them*. They are the true dressage experts. My experience with OTTBs is pretty extensive, however.