I've just started to hit a one-handed backhand. I used to do a two-handed backhand, but i never got any real power behind my shots. So, I decided to convert to the one-hander.
If you never got any power behind your two-hander, it is not because it is a two-hander versus a one-hander. There was obviously a flaw, if not many, in your technique. The problem was not the stroke itself.
Just as you had problem with the two-hander, switching to the one-hander has its own problems. For you at least it seems the problems you're having now are a lot more severe than you had with the two-hander. Pace is an easier thing to fix than a lack of consistency. It sounds like you had the fundamentals with the two-hander, but you don't for the one-hander.
You're asking for advice, but you're not really giving much information about your own stroke. The most important question of all for someone switching to a one-hander is, what grip are you using?
Chances are you're using the wrong grip. I would imagine the rest of your problems are caused by the same thing that caused the issues with your 2-hander and that's the shoulder turn, keeping your head still, and the
proper footwork.
Not being able to handle balls near your shoulder is first and foremost a footwork problem. You have to move your feet to get that ball in your strike zone, just as you do on a forehand. You can't hit every ball from the same position on the court, you can't just let the ball come to you and hit it wherever it ends up. You have to be proactive and put that ball in your strike size. That means either taking the ball on the rise or after it balls (probably far behind the baseline). This is why it takes so much time and practice to to hit the one-hander well, many people never quite figure this out. I think you're expecting too much too soon and jstr sort of touched on this. Learn the fundamentals first.
Do a search on the one-handed backhand. There might be more threads on that in this forum than any other topic. Hell, you've already started two.