Hi Jenna, this can be caused by one or multiple of the following:
1- you may be overpowered, kite might be too big for the wind conditions. try using a smaller kite or going out in lower winds if you don't have a kite for 25 knots+
2- use your trim strap in an effort to keep the bar closer to you and avoid getting pulled forward, this will improve edge control
3- check your line tuning, especially if you just bought a second hand kite make sure all lines are at even length. If your front lines are longer than the back lines, you'll always be overpowered
4-board size: if your board is too big and/or too wide for your body weight, it will be much harder to edge, especially in strong winds and choppy water
5-pressure distribution. the faster you go the more back foot pressure you want to use to control your speed. front leg needs to be locked in straight, shoulders back and hips pointing upwind, eyes looking upwind. If there is too much pressure on your front foot or if your overall stance isn't right you'll be quite limited in your edge control.
6-No matter the conditions, if you have the correct size kite/board for the conditions you should ALWAYS be able to edge your board hard enough to the point of coming to a complete and controlled stop.
check out this video about board/edge control just released by woo sports, should give you a few useful tips:
www.facebook.com/woosports/videos/907228279331181/Christian