Rearing, bucking, nappiness as well as needing "a lot of leg" are all symptoms of your horse not going forward. Many people think that forward only means going forward over the ground. That's the physical expression of forward. That is, going forward is a direction in the same way that going sideways is a direction.
But having your horse "think forward" is even more important if you want a horse that's safe and not exhausting to ride. There are 2 aspects to this concept of "thinking forward."
1. Your horse needs to maintain the energy of his gait on his own so you don't wear yourself out just keeping him going. So, in this case, if he stalls out, don’t give a driving aid first. Just correct him by tapping with the whip or bumping him a couple of times with your legs. When he goes on his own (even if it's only for a few strides in the beginning), be sure to praise him a lot. If he stalls out after a couple of strides, correct him again, and then praise him as long as he’s maintaining his own energy. Repeat this process as often as you need to until he understands you’re not going to "help" him by nagging with your driving aids.
2. If you give a driving aid (leg, seat, or voice), and he ignores you, correct him as described above. Then go back to what you were doing, and RETEST with the same light aid. If you don't "retest", you've only taught him to go forward to the correction—not in reaction to the light aid.
Once you've taught a lazy horse to "think forward", you can whisper with your aids to get him to "shout" his answer. Not the other way around.
Then you have a way to tell your lazy horse to go forward if he wants to rear or buck. Click on Whoa and Go for more info on teaching your horse to think forward.
A Happy Horse