How do fix this? I'm stuck..?

For starts, quit gripping with your knee. Then tip back from your hips and put as much weight as possible into your stirrups. Get someone to lunge you while you ride around focasing on these three things. Its also likely that you aren't putting your legs in the right position. Ride around in two point (at canter) then when you go back to sitting canter try keep your legs in exactly the same place that they were in two point.