Why do I do this, what does it cause and how do I stop?

I have a nice fluid practice swing with a full finish, but when I put a ball there I stop as soon as I`ve hit it, quite often finishing with the club not even past horizontal, and I`m not taking much of a divot so the ground isn`t slowing it much.
I tried to stop my practice swing before it got to horizontal out of curiosity and I nearly broke my thumb.
I`m not playing that badly at the moment (80 & 82 recently off 16) and I hardly know I`m doing it, it doesn`t feel that wrong, but someone mentioned it and now it bugs me.

You are trying to accelerate with your arms and hands only. This will cuase a short finish because from the top of the swing, the fastest point in your swing will be just about hip high. Then you begin to decelerate. At impact, you lost about 1/2 your power...still decelerating.

The solution is to allow your lower body (legs, hips and torso in this order) to fully rotate causing the upper parts to finally get their chance to finish the swing. If your lower body has already done their part, and the upper starts theirs about 1/2 down, then you now have maximum acceration @ impact, and will see the full easy finish all the way around.

Don't mistake what a practice swing does with a real one. Practice swings rarely hit a ball...unless you count that one that you swung too close. Real swings rarely miss the ball...unless you count the time you swung too far.

(All that to let you know that you are out on a limb if you think that the practice swing should be the same as a real one...because when you put a ball in the way of you club, the reality of your actions become apparent and most people will conjour up the past...good or bad. Every ball struck will add to the past...good or bad. Practice swing don't add to anything.

__________________
I'm a golfaholic, no question about that. Counseling wouldn't help me. They'd have to put me in prison, and then I'd talk the warden into building a hole or two and teach him how to play. ~Lee Trevino

I think I do swing too much with my arms, but this was a deliberate strategy on my part to ensure I swing down the target line at all costs - I play a lot on my own in the early morning and so I try out various different ideas, and the most successful one by far has been to make absolutely sure I`m swinging right to the target on every shot, and it immediately took 6 or 7 shots off my average score, coming down from 92 to 85, and I feel I`ll be breaking 80 soon.

The downside is that my swing is over-controlled and a bit stunted as I`ve said, and I`m usually using 2 clubs more than everyone else (not that this bothers me much)
If I go back to a free full flowing swing, my swing path/divot goes left and so does the ball, into hedges, trees, OOB, and I shoot 92 again.