The pressure on the rudder, called "weather helm", is due to some crazy physics involving the center of pressure (a moving point up and down the sail that changes as your trim and the wind changes), your course to the wind (upwind, downwind etc), the drag on the rudders, and, to some degree, your speed. You have noticed that steering becomes more difficult as you go faster in higher winds.
There is a lot of good information regarding toe-in and alignment here but xanderwess came the closest to a correct solution by telling you to see if your rudders are "straight up and down". Xander was addressing the main variable that you can change: drag on the rudders.
Viewing the boat from the side: Drag (weather helm) is induced as the rudder tips move away from the hull and reduced as you "tuck" the rudder tips "under" the hull (towards the bow)l. This vertical alignment is called "rudder rake". Too much rudder rake results in the steering being too easy (believe it or not) and you lose the ability to keep the boat in a firm, straight, line. Too little rake results in too much weather-helm and you have to be Popeye to hold the thing on a course in a good breeze.
How do you adjust it?
Expensive: Buy a pair of adjustable upper-rudder castings with rudder rake adjustment - cams. They apply pressure on the rudder cam and shove the top of the rudder stern-wise, which changes the vertical angle forcing the tips forward, tucking them under the hull.
Cheap: Remove your rudders. Note that there are two holes, one for the pivot and the other where the upper rudder casting connects. I think that the upper casting hole is drilled in the wrong spot. Here is how you change it: Fill the existing hole with Epoxy or Bondo. It has to be a solid fill. When it is dry, drill a new hole a LITTLE higher and a LITTLE further back, about 10:00, right next to the original hole- overlapping the holes is OK, that is why you need a solid fill. A little adjustment goes a long way. An 1/8 inch can change the angle forward by 1/2 inch +/-
Reinstall and sail. Enjoy!
PS- there is a downside to rake adjustment- it gets harder to lock your rudders down so make any adjustments small ones