At your weight a larger rig will certainly help.
Too bad you did not get the Rio L (large) as the extra volume and width would have been helpful
in your progress so far.
As your skills improve, you will probably want a larger sail around 8.5-9.5 m2.
You will probably want a larger fin (50-60 cm would be good) to go with the larger sail.
What issues are you having?
Sounds like you are working way to hard to either control the rig or stay on the board.
Having taught many heavier sailors over the years, my guess would be that you are moving your
feet a little to "compensate" for balance issues.
When you learn to get your feet set so your center of mass stays over the fore/aft centerline of the
board and make your balance and counterbalance (the pull and weight of the sail) corrections at the
hip level and above.
It's a common problem for heavier sailors. It's also pretty hard to learn to "calm down" find the best
foot placement for your weight and your board's volume distribution, and then learn to use your upper
body weight to compensate for the weight and power in the sail (through the harnes).
Hope this helps,