Maximus has given you some good things to try!
A larger board would want a larger sail.
I suggested the larger weed fin (you could even get the 40 cm Formula Duo from Wolfgang) because it will give you alot more "stick" at slow speeds.
If you had enough water, a 48-52 cm vertical fin would be real good at your weight, but it does not sound like you have that much water depth.
I still think a sail with more draft and less top end will get you going sooner and alot easier.
I have both Sailworks Retros (very drafty and great light wind performance but they can be adjusted to give more top end and stability) and Sailworks NXslm race sails.
If I want to get planing early, I use my 8.5 Retro or 9.1 NXslm, if I want more stability I use the 8.5 NXslm.
The race sail just does not have quite as much low end tuning range as the free race.
On the other hand, I can sail the race sail well past the wind speed of the free race comfortably.
Hope this helps,