Fighting the helm
In 10 to 15 knot winds you would prob have better luck reefing to depower the main. First check to make sure your rudders are fully down and tight. If you dont reef, depower the main by tensioning the backstay and tightening the outhaul then ease the sheet off a little. Usually weather helm is experienced going hard to weather, the best way to envision the cause is to imagine your main produceing more leeward lift than the jib, the result being that the stern pushes leeward alternately forcing the bow windward, hence coming to weather. depowering main and powering up the jib will correct this. For smaller results, bringing in the centerboard will help. If you bring it in halfway it will move the center of effort back allowing the bow to come off the weather easier. A good practice of this theory is to sail in heavy weather with the jib only. The results may suprise you, and a 26x in heavy wind under jib only is amazingly fast. Next put up a reefed and depowered main and fly progressively more jib until the boat will stear the desired course with no input to the helm. This will teach you to balance the helm and makes for a much more enjoyable sail.