Oh man, oh man. You asked a doozy of a question. 'Toughness' is a heck of an ill defined term in Western Society, but still hearkens to something we all innately feel. I'll not bore you with a long essay, but give you some resources to start on the journey, should you choose to proceed with earnest.

My biggest recommendation is, of course, google: https://www.google.com/search?q=how+to+toughen+up&oq=how+to+...

For a better site, I'd suggest The Art of Manliness. Brett McKay likely has a degree or 2 in Men's Studies at this point on his site. I like him because he gives good advice, tell you when something is not going to be easy to think about, and he lives what he talks and tracks changes to his advice, sometimes calling out old posts of his as wrong. Here are some links on toughness, but the whole site is great:

http://www.artofmanliness.com/2015/10/26/the-sioux-guide-to-...

http://www.artofmanliness.com/2015/05/07/men-and-scars/

http://www.artofmanliness.com/2014/07/17/keep-your-head-4-ex...

http://www.artofmanliness.com/2013/10/03/you-may-be-strong-b...

http://www.artofmanliness.com/2013/11/04/dig-deep-youre-stro...

http://www.artofmanliness.com/2015/03/12/podcast-105-resilie...

If you's like some reading on toughness and the more historical view on it, no better book is there out there than Dr Carlin Barton's 'Roman Honor: The Fire in the Bones'. Her book is really a masterpiece in manliness, toughness, and much more. Available here: http://www.amazon.com/Roman-Honor-The-Fire-Bones/dp/05202252...

Finally, no real discussion and initial voyage into toughness would be possible without mentioning the patron saint of toughness and hardihood, Teddy Roosevelt. The Edmund Morris Trilogy on our manly lord is the best biography I know of, though I think the 1st one is the best: http://www.amazon.com/Edmund-Morriss-Theodore-Roosevelt-Tril... (bonus points for the Churchill series too: http://www.amazon.com/Last-Lion-Box-Set-Churchill/dp/0316227...)

Hope it helps!