I don't think you know just how much mental strength your team showed out there tonight, and this is coming from someone who values the mental aspects of the sport more than your average.
The mentally stronger team didn't win, the team with all the momentum in the world behind them did.
This was as close as football could get to war without turning violent, those players spent so much more energy than is humanly possible, their legs called it quits before extra-time, but they still kept on going and going. Look at Modric's work on the goal, after 119 minutes of painful agony, how many players would've fallen over and lay dead in his position? He fought harder than his body could ever allow him and made a goal out of nothing.
Now that is mental strength in abundance, and on such young shoulders too. Any human being in any walk of life with that much passion and drive is going to make something massive out of themselves.