without seeing you in action, you probably need to open up your stance, sounds like you are sheeting in hard and racking the rig back to far.
to open your stance,
front leg staight with your foot just behind the mast and pointing forward
back leg bent at the knee, foot perpendicular to the rail of the board near the centreline.
front hand 6-12 inches from the mast so the rig is leaning towards the front of the board slightly.
arms straight.
p.s. don't oversheet.
leaning the sail in front of the centreboard will turn the board down wind.
raking the sail toward the back of the board will turn you into the wind.
this is actually a pretty good stance
where are you located, i'll give you a hand if i can