Tacking in higher winds or any wind requires speed to move through the tack. You have to be able to get some speed up. You also need to be sailing close to the wind... not a reach. Turn the rudder slowly at first and more and the boat slows. Hike out and trim harder as you enter the tack. Keep the sheet tight till the boat gets head-to-wind. Release the main sail, but keep the jib sheeted. Let the jib backwind the bows across the wind. Allow the main sail to sheet out several feet so the boat does not weathervane back into the wind. Keep track of forward momentum, reverse the rudders if the boat begins to back down.
In any case get the bows well across onto the new tack before releasing the jib. Re-sheet the jib first and then the main. Ease the main in to get some forward speed and steerage before sheeting the main tighter.
I had the same problem, and I did everything Matt said except for 1 thing : pushing the main SEVERAL FEETS. Tacking was a nightmare, when facing high wind and waves.
After I started to pushing the main (again SEVERAL FEETS), tacking became very easy, to the point where it is now a lot of fun to tack.