My ongoing learning in Classical Equestrian Arts often forces me to go back to the beginning. It's completely worth the journey. My understanding of 'forward' and 'balance' has evolved tremendously from returning to beginnings, under the guidance of a Master.
Commonly I see riders adjusting the reins to gain a certain 'head down' position or to achieve 'roundness'. I see a lot of busy hands in my clinics and I too used to ride this way. It's tempting to think that these adjustments to the horse to make them relax into a round position will actually relax them into a round position. Repeated hand aids achieve a fake roundness that is observably tense.
Like people, horses get imbalanced. Imagine you lose your balance on an icy patch. What do you do? you throw your hands up and stiffen into a protective braced posture. It's instinctive. Horses do this too! They are not balanced naturally, until they learn it through repeated development exercises, much like a gymnast does for their sport. Horses need development to be able to be riding horses. If they lack the fitness and ability to balance ~ then there is no rein aid that will do it for them.
So what then?
Lunge. The riding horse is put straight and balanced on the lunge, starting at the walk. The imbalanced horse will brace on the circle by lifting its neck high and contracting the spine. The neck will pendulum up and down as they try to balance. Repeated walk on the circle is Yoga for horses, as it develops strength and flexibility, resulting in balance and relaxation. A balanced, relaxed horse will travel the circle 20 times in a good stretch without lifting and bracing. If the horse's neck is still a pendulum lifting and lowering ~ the balance is not confirmed. When balance is confirmed at the walk, then the horse can proceed to the trot and then trot first, then canter.
Horses that are balanced have confidence and relaxation. There is no need to adjust the reins every third stride because the horse is already round, and ready. This is the beginning of self carriage.
Once our horses are able to balance and are supple in both directions and relaxed, we can begin to introduce rein aids that guide the horse through the dressage. Only the lightest aids are necessary because the horse is already round.
Don't throw away the lunge. The Spanish riding school hasn't in 500 years. If you are like me, on a journey to discover lightness and desiring a relaxed, balanced partner consider going back to the lunge, at the walk. Lunge at the walk until balance is confirmed. Then trot, then canter. You will have a different horse.
Lightness starts at the beginning.