How to get a fast horse to slow down through jumps? She constantly speeds up no matter how much I half halt, and how slow I go into the jump.

A case of mental relaxation. Small jump (50cm) transition to WALK about 30/40m after jump, then make a 15-20m circle. Upwards transition into trot on the circle until she relaxes, point her towards jump but the moment she starts pulling towards the jump then you QUIETLY transition her to a walk, talk to her, and make that circle again until you can feel her breathing and relaxing. Then face jump again and slowly trot. If she pulls quietly transition to walk and then WALK her over the jump. Until you can feel she is waiting and relaxed. Now... your job is to slow everything down. Stay off her mouth. Half halting incorrectly can actually have the opposite effect:pulling. Try and keep transitions as slow and relaxed as possible. If you feel yourself getting anxious then start singing. Helps with breathing. Put a neck strap on so that as you go over the jump and there is a chance you are left behind then you can grab onto the neck strap. It requires IMMENSE patience. But you can't enjoy jumping until both you and the horse are relaxed going over a pole OR a cavalletti. Let me know how it goes.
I think you need to bring it back to basics, I read a very good article in my magazine about this! They started off with just poles on the ground and walking over in-hand, halting every now and again. Then built up to trotting in-hand. If they're not barging then you can try it on horseback then. Then when they're comfortable and not rushing do the same with small jumps. And halting immediately afterwards.
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