I have a welsh cob who in the school can cause similar problems in some ways. He wants to do everything at speed & would love to grab the contact & run with it if he ever had the chance. Contact has to be there & his frame short & round enough that he's stepping through rather than running downhill, but oh it has to be so elastic & forward so he doesn't grab it. Sit up too much & he hollows & dashes off, but too forward & he's got me over his shoulders & we're off - hands up helps reduce the urge to go too forwards though. It's so important to keep the leg always there because otherwise a wobble means he's off - leg off is for lazy horses who need a wake up call & that really doesn't look like your girl!