Squeeze really really hard with your knees and half half and give constantly. Short half halts though and when you give, let the horse have a second to respond and slow down. If he is about to speed up then catch it before but make sure you don’t mix up speeding up and when you give. After you give really make sure the horse is definitely going to speed up because if you pull when they relax it’s not rewarding and they won’t ever wanna give. Use your body too. Tighten your core and don’t follow the motion and say whoa/easy or whatever vocal cue you use.