Flexibility in horses is fundamental
Its pretty simple, if you don't have flexibility, you don't have control. This is where fundamentals are very important. Flexibility in horses is fundamental in every discipline of horsemanship.
Flexibility begins with lateral flexion or the ability to bend the horse's body with one rein. This is an exercise that has to be habitual for you and your horse.
Everybody knows how to do lateral flexion but they don't do it enough. And a lot of people think they know how but they really don't. They don't know how to feel the difference between resistance and softness. They don't know how to tell the difference in a bend and a pull.
Make it a habit to use one rein
When you use one rein it allows you to position his body and to build flexibility. If you use two reins, you will pull the horses head and mouth, his natural instinct is to pull back.
When using one rein at a time, its harder for your horse to brace or lean against you. Just hold your pressure until he finds the release point in giving. Then he learns to give instead of a pullback. That's how your horse learns to yield to pressure. The horse learns to "follow the feel" on the rein, not because you yanked on it. If you have to use more than 4 ounces of pressure, to produce the bend, then you don't have lateral flexion. You have to continue until he is soft and supple.
The horse is saying he needs relief.
People are always having problems with resistance and its because they don't have the softness in the neck. The horse is also telling you he does not trust you and that the bit is hurting him.
Without lateral flexion the horse the horse cannot truly be "on the bit." Without lateral flexion, none of the lateral movements are possible. Even a simple turn or circle can be difficult without lateral flexion. The braced or resistant horse will lean to the inside and pull his body over the ground with the front legs.
If you feel resistance then go back to lateral flexion until you can easily bend the horse on his side with one rein.