Hmmm. I can't explain this for certain, but here are some ideas:
1) The actual weight of the sails and spars is about 20kg, usually a lot less than that, and yours are light ones.
2) The height is less than 7 metres, but the centre of mass is probably less than 3.
3) You plus your dad have got to be more than 100kg.
4) Standing on the board your lever length is probably about half a metre.
5) If you have trouble bringing the boat upright, this is down to the lever keeping the boat down exceeding the lever lifting it up.
6) Rig lever = 20*3=60kgm
7) Crew lever = 100*0.5=50kgm
Suddenly it is plain why you have to lean back a bit!
9) You said you and your dad were hanging on to the board. If you weren't standing on it, not all your weight would have been pulling down on it. Any part of you that it is in the water will be floating and not pulling down. If you don't believe me ask Archimedes
http://en.wikipedia.org/wiki/Buoyancy (watch out though - this page has a mistake in it under 'compressive fluids')
10) Dynamic effects. When there is lots of water around, even quite a big force will cause v slow acceleration which may be hard to notice. This means standing on the board and waiting a long time may help.
11) Kicker and jib sheet can cause the sails to hold a lot of water. Let these right off, as Born Slippy said.
Keep at it - and come to the nationals for maximum exposure to all kinds of high performance sailing techniques, overlaid of course by a generous helping of sensible and responsible behaviour.