You've got weather helm
Sounds like your boat is out of balance, which is pretty normal for Macs in moderate/high winds. Easiest thing to do is ease the mainsheet. Increase the jib area if you can (ideally with a furler). If you're still horsing the tiller, you need to reef the mainsail. The science is as follows (skip this paragragh if you don't care). A sailboat basically pivots around the keel/centerboard. The mainsail is behind the pivot point, so it is always working to turn your boat into the wind. The jib is forward of the pivot, so it tries to turn the bow away from the wind. When these two rotational forces are unbalanced, the rudder must compensate to steer a straight course. In your case, the mainsail is significantly overpowering the jib, and you have to pull the tiller hard to windward (helm to weather) to stay on course. Designers build in a certain amount of weather helm so the boat will automatically round up in a gust. The other option (lee helm) can be dangerous and spin you broadside to oncoming seas or even around to an accidental jibe. An interesing exercise is to practice steering the boat with just the sails by filling one and luffing the other. Use the mainsail to turn to windward and the jib to turn to leeward. Good luck!
