F1 isn't just a mental game, it is a physically demanding sport.
See Schumacher's comeback. You couldn't fault his motivation and dedication, but the ability just was not there anymore.
This happens to boxers as well, with Sugar Ray Robinson highlighting it in his book. He knew what he wanted to do, he could see the opportunity for a counter-punch or he he could see a punch coming and knew what he need to do in order to avoid it, but the reflexes were just not there anymore. Hence older boxers tend to become more stationary and take more punishment, the body just won't let you do what it done 5-years earlier.
In a sport like F1, if you lose a tenth on your reactions, that's precious time on track and Father Time starts taking those little things from you. Another thing that happens to F1 drivers , and what happened to Hakkinen, is you maybe start second guessing yourself. Would prime, fired-up Hamilton not have taken a risk in trying to pass Gasly? See the ballsy move Russell made on KMag at the chicane whilst Ham just followed Gasly around.