Transitions, transitions, transitions... with a young horse, I firmly believe the most important things are good brakes, a responsive gas pedal, and the beginnings of a good work ethic. With my horse that I broke out this spring, we are around our 10th ride and are still focusing on whoa, walk, and trot. Once she becomes balanced and confident going between those gaits, cantering will come into play, but until then we are just working on relaxation and responsiveness. I'm not expecting perfection with steering, but we are incorporating turning around and serpentining through obstacles; I will worry more about more advanced steering once she becomes more balanced and relaxed with a rider.
On the opposite end of the spectrum with a well broke horse, transitions can always use a refresher. My retired show horse is now mainly a horse for my boyfriend to learn on and trail ride, but every few rides I hop on and remind him that I still expect responsiveness despite mostly being ridden by a beginner. If I don't remind him once in a while what is expected still, he isn't going to remain the ideal steady-eddy for my boyfriend.
It sounds like your horse is between both of my horses; I'm assuming she is comfortable with a rider enough to not be considered a greenie, but she still has things to learn. Some things she may need just a refresher on, whereas others she may need to be taught. I would start with simple transition work; walk, stop, trot, walk, stop, trot, stop... This can be done in the arena or on the trails if you are comfortable. Once she is getting transitions alone, start doing them around obstacles; walk around the barrel, trot to the tree, stop, back three steps, do a figure eight between cones... Really the possibilities are endless at this stage, and if you keep changing things up and changing the scenery, she probably won't get too bored and neither will you. While working on these things, start teaching her how you expect her to be ridden, whether it be mainly off of seat for brakes, legs for steering...whatever you would like.
However, one thing you could change is that if she has issues steering with a plain d-ring, you could try a full cheek snaffle instead. I usually do my first couple of rides on a horse in a rope halter, and then step up to a full cheek, and eventually into a d-ring. If you ever consider showing where a shanked bit is required, the time to begin introducing neck reining is now, but I personally don't worry about it because I vastly prefer english competitions and gaming events that don't have requirements.
Toofine - 1998 Half Arabian
Minnie - 2013 Morgan