I seem to do both, miss the shot or hit it to hard
Without seeing your stroke it's tough to diagnose the problem. But I suspect you may be off line. The more off line you are the more difficult it is to keep your stroke straight. I would suggest finding an instructor or at least a very knowledgable player to adjust your stance, arm position etc.
As for over stroking the ball, do you take full strokes on every shot? Or do you shorten your stroke for soft shots?
Here are some things to try,
Place the cueball on the foot spot, shoot the ball up table and try to have it return to your tip. Now check to see if your tip gone past the spot, if it has obviously you've followed through the ball.
Again with the cueball on the foot spot, set up some long straight in shots. Since there is no aiming invloved here, it is all about your mechanics. Just focus on making a smooth stroke and keeping your head still.
Do you have access to a snooker table? If so here is a practice regime I use,
Place a ball on the black spot, take cue ball in hand, pot the ball and play position to shoot another ball from the spot. In order to keep the cueball around the spot you need to develop a good touch. If your poking at the ball you wont be able to spin the ball as accurately, and you will lose position quickly. Try for a run of 5, then push it up to 10 and so on.
It's also worth placing a ball on the blue spot, the cueball on the balk line for some long straight in potting. This is a great test of cueing, and can be very difficult especially if your not playing well.