The trick to riding one handed is understanding the two inputs you are giving the kite - power (sheeting in and out) and direction (turning left and right).
Assuming you are cruising, when you are sheeting in and out you generally to not want the kite to be turning. From you post it sounds like you are sheeting but by doing so you are also initiating a turn in the kite. The turning is caused by the force not being balanced on the bar when you sheet in.
When you ride with 2 hands, you sheet in and out by applying the same force to both sides of the bar. When riding with one hand you need to do the same thing.
There are a few ways to do this, either by putting fingers on both sides of the chicken loop and/or by twisting the bar in your hand.
When twisting the bar, if you have your right hand on the bar (back hand - like in the picture below) you want to twist the left side of the bar towards you.
Hopefully that makes sense... good luck and don't give up!