it's you!
but don't worry because you're doing what everyone does at this stage.
yes it would be easier with a wider more modern board. but.....
easty has the right advice. because you are not loading up your harness you are weighting the rail. that is turning your board into the wind and stalling the tail.
could also be your boom is too low. your boom should be around chin height.
get planning along hook into your harness.
once comfortable move your feet back along the centreline of the board.
then place your front foot in the front strap and practice that for a bit.
try, to keep the board flat. ie. try not to push down on your feet which will result in carving the board to windward.
once you've got the feel of front foot in then slip your back foot in.
you'll know when you are doing thing right as you'll start to get catapulted. this is a sign you are getting your weight off your feet. from there you will learn to use your weight to handle the sail and stop the catapults.
also check out this