hey, i had the same problem in the beginnig.
The first problem are your harness lines: rather then moving them closer together, move the entrire thing about 5 cm to one side or the other, most likely towards the clew of your sail. Keeping each side of your line about 1.5 fists apart (don't try to put them closer then one fist appart, you lose a lot of controle and gain unnecesairy powe which you won't be able to controle properly)
Second problem is your position, you lean in to early first of all and when you do, i think you put too much weight on your back foot which will make you lose your plan and have to go oversheet to pull yourself up again. Try to move your front foot somewhat more inwards, more towards the mastfoot before the plane, once you can fell it coming first lean hook in your harness, lean somewhat back, move your front foot outwards, then bend through your rear legs knee keeping your weight on the front of your board and leaning in at the same time, this should set of your plane right asway because by doing this you tug the mast over the nose of your board, you open your sails and your wieght displacement is ideal. Whilst you're doing all this you should slowly be moveing more and more to the back of your board and try to maintain this position untill you feel fully powered and then your can alter your course to where you want, strech your legs if you can and lean in. Always look in the way you want to go and keep your front knee pointed towards the sailingdirection (it sounds weird but you do it automatically)
I hope this helps
ps: be carefull with the big boards in strong winds .... they can handle to be overpowered but you need to keep them as such else they start to catch wind and waves which can make surfing hellish
Haiko, AKA crazychemical