I'll reiterate some of above, your paddle needs to pull paralell to the centerline, not the rail. Keep the shaft in at midships. At my feet, before the draw, the inner edge of my blade is under the board.
Try weighting the opposite rail a little.
Big fin, all the way back in the box.
And finally, make sure you are not too far back on the board, a common beginner mistake. When in doubt favor the front weight. Having the nose high will allow the board to spin. Good around the mark, not in a line.
You should be able to paddle on one side as long as you want. I can paddle to exhaustion on either side. Switching sides just to steer is unnecesary and slow. It takes a while to finesse a slight draw stroke on the catch, but it can be mastered.
Keep working on it.