If you're not first continuously training and cultivating strong mental resilience, all of your attempts to learn and read and study and get smarter and gain more knowledge and understanding will ultimately keep failing and will largely amount to nothing.
Intelligence is about how accurately you can perceive, how much, how well and how efficiently you can filter and process information to reach accurate conclusions, how well you can translate different cues to different courses of action on time etc.
Psychological strength or mental resilience is about how you can protect and shield all of that from whatever would or could cause interference or hindrance to it. It's a huge aspect of intelligence in itself.
I mean it most broadly. Even when you're dealing with very advanced AI intelligence, if it can be easily made ineffective or faulty due to e.g. some electromagnetic disturbances or a weather condition, easily made to malfunction or easily manipulated by someone with ill intent, that's a huge problem. Same with human intelligence.
The point is to be as resilient and psychologically unshakable as possible. To the degree you're not, your own errors, random events, problems, wrong inputs, wrong outcomes or wrongly influencing people can throw you off. If you're thrown off your game, all of what you've learned, trained, prepared for etc. becomes useless or substantially hindered.
The point is that it is one of the most important factors to achieve whatever it is that one needs and wants to achieve. It's as important as intelligence itself. There's no point in building a vehicle if it's going to easily crush in rain or catch fire under the sun or just be a generally fragile thing that easily breaks. One needs to be resilient af.
It's like you're on the road, and then there are all these things that hit you from the outside and inside, and it's more important that you have the strength and tenacity and solidity and clarity to stay on the right road and drive towards the right direction and not be pushed off of it or blown up or stopped from the inside.
It's even more important that your way of navigating is intelligent about weaknesses and holes in your design and doesn't drive its carrier into the mud if that is its krypton, and doesn't leave it exposed or easily abused for people who might intentionally or unintentionally do harm. That involves all the internal factors and choices about shaping one's schedule, environment, people to hang out with, learning resources and so on.
I mentioned things that can hit you from the inside. The internal things that can be triggered by some factors and pop up to derail you in ways you expect or don't expect. Consider one of the universal issues - problematic emotional imbalances such as anger, frustration, raging, getting upset, sadness or being overly sugar-like, overly optimistic, overly all roses & rainbows, overly serious, overly worked up, overly excited etc.
These things are like mini-bombs that are launched when certain triggering criteria are present. They hit you from the inside and can quite literally shut down your brain, mostly. Getting angry, yelling, voicing utterances of frustration etc., are examples of what acts to quite literally shut your brain down, mostly, and make you unable to effectively solve the problem at hand.
These things also often occur as persisting patterns instead of really rare occasions for many people. They are often further entranched by some interactions, in one-sided or mutual manner, where some people know what the triggers for these mini bombs in others are, and are inclined to work on these triggers and get a lot of these mini-bombs launched for mostly strategic or rather non-strategic reasons, fairly consciously or mostly unconsciously.
So what that you have all the knowledge and understanding and experience and skill and foresight and prescience and vision if you get spun up and imbalanced versions of anger, frustration, depression or some other derailing forms of crap set in? Your brain's shut down, you're getting very busy shouting out or doing some intense and equally unintelligent or harmful things.
So what that you study or train or work super hard every day if someone or some group of people can say or write or do something or treat you in certain ways, and suddenly you're totally thrown off your game? The world is full of people who intentionally or unintentionally influence in all kinds of ways that can cause you huge derailments and harm if you're adapting poorly to it and your mental resilience isn't handling it well.
So what that you keep working hard and stay on the path if you keep disrupting yourself and getting distracted when you make an error or do something wrong despite having practiced it a ton of times etc.? Your brain's shut down to a high degree, you're gonna fail to assess the scenario correctly, it can easily lead you to map problems and solutions inaccurately, which can easily lead to a cycle of wrong assessments, wrong solutions. This creates mounting frustrations which generate even more inaccurate assessments and ultimately, even severe mental and emotional disorders. If one lacks mental resilience and psychological strength, that is.
In essence, mental resilience and ability to stay psychologically impregnable in the face of all the things that can exert significant or strategically problematic pressure one one's psyche to not get pushed off course, stopped or entangled in some non-aligned things, is paramount. One needs to be as effectively clothed in resilience and psychological strength to stay on the right kind of direction and destinations, as possible.