This is not an ultimate forward head posture fix (because usually it needs a systemic approach), but longus colli definitely needs strengthening. Very often this muscle is weak in the most of population.
Strengthening deep neck flexor muscles (longus colli) also helps to avoid constant tension in neck extensors, because of reciprocal inhibition. Just the same way as when you strengthen your glutes to help your psoas muscles relax.
You can buy these mini bands if you don’t have yet.
