Circles!! Have your horse focused on you and not about getting away. Keep his attention! Try doing a circle and then getting smaller and smaller and then bigger and bigger to keep his attention! If that doesn't work try having someone lunge you for a bit. Just stay patient, calm and talk to him!
You could try different things like circling or halting and backing up. We have a horse at my barn who does this and we just have to halt and back up before going forward which tends to help. Also to keep them from speeding up while cantering, you can circle and/or do half halts. Hope this helps! xx