I'm not sure if the '83 Lightning was the old diamond tail or the pintail one we had in '85, but if it's the latter you should be able to sail that gear in 20 knots+ with one hand lightly holding the boom and everything nicely balanced. It's hard to diagnose trouble without seeing you sail, but the issues could be;
1- not using the mast track. It has to be all the way forward when sailing upwind in strong winds.
2- incorrect stance - shortboarders often stand a long way back all the time and if that's all you have around you, you may be copying their stance. That year the F2 Lightning had no "beating straps" to use upwind - if you are using the straps when sailing upwind or in light winds, you are too far back and that will cause the board to turn upwind.
Upwind on a Raceboard you must stand much further forward on the board, with the centreboard down and the mast track forward. See Anders' upwind stance and setup from 53 to 1:09 in this vid;
and compare it to the reaching and downwind style and setup in the rest of the vid. With the '85 Lightning you will be standing even further forward, from memory.
See also the boards going upwind from 1:30 to 2:30 here, and the very different stance and setup to most windsurfers;
The sailors are standing well forward, near the centreboard which is fully down.
As noted above, use plenty of downhaul - although if the rig is an '85 one it doesn't need as much downhaul as a modern sail.