Roger probably could answer this better, but I'll give a couple tips.
You wrote: "I am not yet getting into the straps or harness." That's the main problem. You are probably standing close to the mast base and trying to control the power of the sail purely by sheeting in and out with your back hand. That's a recipe for exhaustion and getting bounced and catapulted. To sail with control in a lot of wind, especially with a wide board and long fin like you have, you need to be fully committed to the harness and footstraps with your body hiked way the heck out over the water, and the sail leaned to windward like this:
Or as a friend of mine once put it "keep the hammer down".
I have some advice about using the harness and footstraps in this blog post:
There's a couple things you could do to make it easier for yourself. 1) Make sure the footstraps are in the inboard and forward positions where they are easiest to reach. 2) Use a much smaller fin if it's more than 20 mph (about 42 cm) 3) When it's windy put your front foot in the strap right away, even before you are planing, to avoid getting catapulted. With the proper mast-base pressure and your back foot over the centerline of the board you can do this without rounding upwind. 4) When you're planing good with your front foot in the strap, the sail fully sheeted in and your body leaned out over the water, you can get in the back strap. Then, while still "keeping the hammer down", you can thrust your pelvis forward, hook into the harness, and then carefully push the sail away from you with both hands to transfer the power into the harness.