Horse strong at competition

[Dani1978]

Hi. My daughters horse is an ex racer. He’s 11 and is great when schooling and in the warm up area. When he goes into the ring to showjump he turns into a tank! He doesn’t do it any other time. She jumps him in a Neue Schulz tranz angled lozenge bit but he still manages to take a hold and drag her round out of control sometimes. He clearly loves jumping but my daughter needs more control. Any advice would be great!
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