One of my kids had some pretty tight hip flexors and is only 5'3". She could run like the wind, but not raise her knees very high. So, it only figures that she is the one that goes into polo horses big time. Tall, often needing to mount without a block available.
I had her lay on her back and pull her knees up, one at a time, using her hands. She eventually went to doing a crawling, lunging thing on the living room floor to get more hip flexion. Bending one leg forward with the other extended back and laying forward onto the bent leg.
The is a saying in exercise/rehab circles, too. It's "Stability before mobility." So, any exercises that will strengthen your trunk and give your extremities stability will also help you control your whole body as you mount a horse. Pilates, yoga, some plain old army exercises, lots of options.