Hi Sam, and Jeff....
Gloves (any size gloves except the palmless kind) really add to the diameter of your boom.
Also, when the weather gets a little colder and the wind comes up a bit there is a tendency for you to bend your arms and get in a little closer to the rig. This causes the pull from the rig to transfer from your harness and the big muscles in your hips and thighs to the much smaller mucsles in your arms and shoulders.
Also, the higher the windspeed the more likely you are to "gorilla grip" your boom.
Learn to use your hands as simple "adjusters" and try to relax your arms and forearms so that you can "play the piano" with your fingers on top of the boom.
I know this is hard to retrain yourself to do. I still tend to tighten my grip when the wind gets stronger and the water/weather gets a bit colder.
Also, the cold causes arm fatigue.
So, there are a variety of factors.
Ist) Get the smallest diameter boom yoiu can find to use with your gloves in winter or cold weather sailing.
2nd) Learn to trust your rig to hold you up so that you don't oversheet, and learn to anticipate the lulls so you are already moving your weight inboard as the lull takes the power away from your sail.
3 rd) Try to relax your arms. Think of them as simply "ropes" that connect your shoulders to the boom but do not take the load of the sail pulling. Use your arms in conjunction with well balanced harness lines so that you use them to simply make adjustments. The harness takes 90% or more of the actual force generated by the rig.
Hope this helps,