I hate to say this, but you just have to force yourself to reduce the backswing.  You could have someone watch you and let you know if you are deviating from what you want.  I think also, positioning yourself at the optimum points will give you a feel of what that position should look/feel like.  For instance, if you know where the ball needs to be at the top of your backswing, hold that position and get a feel for what that feels like.

Ultimately you should see a respected coach to get detail goals.  You can even send video to some if there aren't any nearby.

A video would show a lot. Absent one, there are 3 elements you should focus on.
First, make sure that as your arms descend into your "backswing", that:

1. Both of your forearms form a straight line (180 degrees) from the tip of your
    right elbow to the tip of your left elbow. Maintain this position
    through your release .

2. Make sure that your left elbow does not go past your belt buckle at the top of
    your backswing.

3. Your forward spine angle should not be more than approximately 70 degrees ( from
     a vertical position).

Hope this helps!

You are very welcome. I am happy to hear that everybody's suggestions have helped you. Just to be clear, using the left elbow as a guide in gauging backswing height
seems to be a good "measuring stick" for many successful 2-handers. In most cases, the left elbow usually will not go past the belt buckle in the backswing. If it does, it will sometimes force the arms to break from the 180 degree position, causing alignment problems on the downswing and release.