A couple of tips that may help.
First, do you have small hands with short fingers?
If so, what is the diameter of your boom.
I had what was diagnosed as carpal tunnel years ago, (even using a harness, but with big sails and a large diameter boom).
I sailed with a doctor and she took a look at the pain and numbness I was experiencing and told me to
take some Advill after sailing, and get a smaller diameter boom.
She was correct. No surgical procedures for carpal tunnel. I've been sailing pretty large sails for 20 years
with only minor problems.
Also, as a beginner, you probably are holding very tightly to the boom with both your hands.
Loosen up...... just drape your hands over the boom so you can "play the piano" with your fingers while
You will pick up harness use eventually. Just stay with it and it will come to you. You will get tossed over the front a bunch of times, but soon you will learn to anticipate the gusts and sheet out.
Are you using a roller harness hook (Reactor Bar).
This makes it much easier to ease your sheeting angle in gusts without losing the rig.
The 51 cm fin is most likely too big.
Pick up a used 34-40 cm weed fin if you have weeds or shallow water to deal with.
Wish you were near Cape Hatteras (not today with Hurricane Irene bearing down) as I think we could
get you staying on the board alot more of the time with a few minutes of instruction.