Mut-the reason you were going over vert is that you were leaning back.
When doing a double pump, you should stay just a tiny bit forward from a neutral sitting position in your boat.
The most important part of the move is the second part. If you truly master the second part (the down stroke), you don't even need the forward stroke any more.
For your "backstroke", think of it more as a down stroke. our paddle(both blades) should be completely flat on the surface of the water. Without leaning back, turn your entire torso until you are completely facing the water, then push your stroke down.