The only Mental Toughness feat I'm aware of is from Dungeons and Dragons Online: Stormreach.
In that game they use spell points - a 1st level spell costs 10sp, a 2nd level spell is 20sp, etc. Metamagic feats increase the cost accordingly. Mental Toughness in turn gives you +10sp to your pool at 1st level and +5sp each time you level up.
It does not, to the best of my knowledge, have any analogue in normal D&D.
And frankly, nor should it. It would be one of those no-brainer feats that you're warned about. Every spellcaster and their mother would take it because what spellcaster wouldn't want a single feat that gives them more spells/day and scales up as they go?