Self-satisfaction : No matter how much you try, greed always triumphs.
Standing up for your beliefs: External pressure and lack of determination and guts makes it very difficult to ignore others' advices and don't give a fuck.
Not being jealous: No matter how much people rant about how they never get jealous, deep down everyone feels let down by himself.
Become a master of something: Although this sounds easy, things are very easy to learn and very difficult to master. From chess to tennis, writing to speaking, dance to music, any and everything on this planet is very difficult to master. And if you say you have done it, you are probably just competing locally.
Keeping your commitments: It is very easy to commit (for some people) but respect for those who actually keep their words.
Emotion vs Ambition: Your best friend (who has stood by you through all the springs and autumns) has just gone through a very bitter phase of his life. And you have a very important job interview at the same time. If given an option to chose only one, what would you do ? This is a very very hopeless scenario and I hope, no one comes across it. Ofcourse, there are people with emotions as their first priority and some with ambition as the same. But the point is, its still very difficult for a normal, balanced person.
Realizing that there may not be "happy endings" in our lives: Our films (Bollywood) has fed this concept of "happy endings" in our mind so much so that people refuse to acknowledge the fact that there may not be a fairy tale ending in everyone's life. Be it career, relation etc, accept that there are roadblocks and start out on a different path.

https://www.quora.com/What-is-a-hard-thing-to-do-in-life