Jumping too hight isnt a bad thing at all. Now there is good ways and bad ways of doing it. Ur horse can jump high but flat or too open, or high and completely out of rythm and all, you don't want that. You need to work him on obstacles far from each other, put polls within a good distance to the obstacle calculte with your steps about 2 and a half big step for trotting, and four for cantering now it also depends on the size of the horse etc.. but if you put several polls down on the ground before the horse jumps it will help him to structurate his jump. Also do circles to have him listening to you and too have him well focused.
Now on the other side, and as every one knows it oftens come from the rider. You need to have good balance for any kind of riding you will be doing. You need to ride that horse without stirrups or even saddle at least 3 times a week an hour each time, it will make a giant difference to you and how you feel the jump later, it is very important!
If you have a picture of your horse jumping it would be great and would make it easier to see what could be the pb.