Mental Toughness

Garry Kasparov once said it is important to be mentally tough to be successful at chess and that you must practice this not just in chess but in all areas of your life. How?

you could start by not asking people on the internet how to be mentally tough.

Place yourself in positions that you are normally uncomfortable and make yourself comfortable :)
[Hiding your feelings doesn't count]

Live free or die

Mental discipline: eradicate all conceit, prejudice, superfluous thinking... Shadow Work: figure out your blindspots and self-fulfilling prophecies... Train your mind to see clearly and objectively as possible... learn empathy and stop projecting your hang-ups on your opponents...and finally: Place yourself in positions that you are normally uncomfortable and make yourself.

Play bad openings like 1 e4 b5 or 1 e4 f5 and give your best to win.

G.K. maybe meant that be "mentally tough to be successful at chess" can help you to find the best move because you are in peace with yourself,you are optimistic. Another example is former world champion R.J.F.,he had a lot of trust in his possibilities. If you do not believe in yourself,none will do that! Of course a bit of humility is never a bad thing. Look at the glass like half full. You prepare an opening trap against an opponent,game does not go like you though. You must have the force to overtake this moment and be psychologically ready to go on to fight at high level. In life (quite) nothing goes at 100% like you had planned.

I need to completely disagree with Archiduxus and say that the internet will be one of your best resources for finding how to become "mentally tough". Look up (on google or wiki, whatever you like) things such as mental discipline, time management, prioritization of goals and desires. Mental toughness isn't something you can just suddenly gain (except in edge cases) but rather learned through the course of your life. Start now if you can.