How good is your independent seat?
Ideally, you will be able to sit quietly on his back, just letting your legs hang without gripping, and not jerk him in the mouth when he launches forward.
I am glad to hear that this has been recognised as an issue, as I hear a lot of people commenting on how forward and reactive to the aids their horse is - when it's not forward, and it is evading the aids by launching itself forward.
As you have him in walk, relax your seat and allow it to swing with the walk motion. If you're legs and seat and really relaxed, you'll feel that your legs will want to be applied in a 'left, right, left, right' motion. Go with it. Allow your legs to hang against his sides (think of a wet towel, or a slab of meat over a rolling pin), take deep, slow breaths and just relax as much as you possibly can.
If he goes to launch forward, keep your seat gently swinging in walk motion, and just use your seat to bring him back to the rhythm and temp that YOU want.
As the rider it is your job to dictate the speed - doing this clearly will make the horse more confident in your capacity as a leader and you will find that it becomes more willing to comply.
The same thing goes in trot and canter, just keep your seat going at the speed you want - preferably a fairly slow tempo, and let your legs hang down his sides with no tension, just constant light contact.
If he goes to rush off, don't attack him, just gently and quietly bring him back to the pace you asked for and keep going. It won't take long for him to start coming to the realisation that running off will not take the pressure off. He'll learn to live with it and not be so electric in time.
I'm working everyday with my instructor with my independent seat, have been told that I sit very passively with him. But I have also been told that I should NOT be sitting passively constantly as it will make him furthermore precious with where my legs are, and timing. I'm planning on doing showjumping (he does love his jumping, and is pretty calm right now with it) and I need to feel safe enough that if something happens which makes my leg slip back or myself to lean forward more that necessary, he will not panic or get flustered.
He goes well when you're balanced, and being very quiet. But I've seen other people (including myself!) that he takes off on the second they are unbalanced. Not nasty taking off, just running on and losing any brakes.
Your advice is extremely helpful, because I keep getting told so many different things (With most people its their way or the highway ;) ) And you're saying what I am thinking.
Its so easy to get frustrated dealing with him when he is having an off day, to avoid any of this frustration I take him on a gentle hack, its not worth making us both uncomfortable and ruining a weeks worth of training.
Do you think there is an age where retraining becomes very difficult?