^^^ What Karina said lol.
French links and “curb” bits are good
If your horse is soft in its mouth but becomes heavy just in certain pace simple snaffle d-ring with one or French link should be enough.
You should have a look at Exercises like transitions and changing reins and circles to help and encourage the horse to use its hind-quarters more.
Also you could add cavaletti, grid work and going uphill and downhill on cross country course will help in concentration and keeping their mind busy and thinking forward.
These things will take your horse off its forehand and it will automatically make its head lighter. 😄😄