I need horseback riding advice please!!!?

So I used to ride full on Saddlebred at a Saddlebred barn. My horses had show shoes on at a time and they were wonderful showers, but as my family and I read more and more about horses, we realized that keeping them cramped up in 11ft x 13ft stalls all day was torture for them. They needed to be in the pasture a big portion of the day for them to live longer, healthier, and happier lives, so we moved to a more relaxed hunter jumper barn that turned them out everyday and gave up lessons. I have not had a lesson in almost a year and am getting into a bad habit of coming a lot but only riding maybe once every 2 or 3 weeks. I rode my pony while my sister rode her horse yesterday. My sister rode fine, but I was terrible! My legs were weak and I kept getting flung everywhere in the saddle! Also, my pony kept MAJORLY cutting corners while cantering (causing her to look like she was barrel racing with her sharp turns!) and getting away with spooking and flinging her head up. She also knows how to slow-gate/ rack but I am afraid she will lose that talent if I don't practice it! Also everytime I ride her I get sooooo frustrated with my riding and her fast swerving and spooking. Someone pleassseeeeee help me!!! I don't wanna have her lose her special Saddlebred talents! What do I do to get her to listen to me and not get away with everything? What do I do to get my Saddleseat form back?? PLEASE HELP ME I'M DESPERATE HERE!!!!