How about some long and low work? At the walk, trott, and canter, ask the horse to relax and reach. Do collection work. Do trotting poles. Layout 4 ground poles on a large circle. Do lots of transition work. Also, something I find very helpful are transitions on a 20m circle. Ex. Do about 6-10 strides at the canter and ask for a trott, do 6-10 strides at the trott, and then canter, then halt, then walk, then canter, etc. Make sure you do this in both directions. Increase your time slowly and don't ask for too much too soon.