Your problem is that you lack the muscles to ride at this level.
You need to stretch, like a dancer does, no bounces, just hold the stretch and relax your body--relaxing your stomach helps to relax the rest of your body. Straddle and stretch left, right and center. Do this on the floor at home. Use the carpet, or use a mat, or even your bed. Stretch your calves by using a step. Stand on it and lower your heels. Climb the stairs slowly, stretching your calves by lowering one heel at a time with every step.
You need to focus on your legs bc lack of leg stretch makes your back muscles work in a way that they shouldn't, which is beyond JUST holding your torso up. Back muscles can be easily thrown out. 5 pounds of weight will do it.
Learn to do "plies". Demi-plies work well, but then you graduate to grand-plies. Even though we ride toes forward, do the plies in turnout. Strong legs make a strong rider.
Also, learn to do crunches without putting your hands behind your neck.
RE: repetitions--however many repetitions you can do TODAY, is the # of repetitions you will try to do every day or, at least 3x/week. THIS is the secret to creating a 15-20 exercise habit, instead of climbing a mountain of more and more repetitions bc you will become discouraged and give up.
THIS will help you. =D Equibarre with Angelea Kelly Walkup: horsebooksetc