Long and Low.
Make the reins really long and then spread your hands as far as you can
Pretend your reins are like 2 sticks connected to the bit
The idea is to get your horse to bring his head out and bring his nose under his knees.
Start this at the walk and then the trot.
Once you start this at the trot begin to only shorten the OUTSIDE rein while on a circle and place it on the withers. And keep your inside rein loose, it doesn't nothing. And use all leg.
You can do this over trot poles as well.
Make sure you sit up very straight with confidence and have a lot of leg.
If this is confusing let me know and I can explain it better if needed! It helps a lot and it also helps with the horses footing and gravity.
Be patience cause this moves the center of gravity forward to where it should be and many horses aren't used to it, be patient and give a lot of praise!
cavaletti/pole work(ground or raised), hill work (up, down and across), transitional work, ground strength and stretching exercises, lengthening and shorting the stride at a trot and LOTS lateral work and figure 8's! Good Luck and remember it takes time