The toughest thing to do in life:
Conquering our own Mind ("मन"):
Mind is very fickle(चंचल) and unstable in nature. Our mind is the generator of all kind of emotions, some of them are positive and some are negative.
Fear, Envy, Jealousy, Hatred, Pain, Sorrow, Lust, Emotional attachment and et cetera. Unstable and disturbed mind generates such negative emotions which causes trouble in our life.
We continuously hear and read that, We need to conquer our self if we want to get rid from troubles. But is it that easy? No. It is the toughest thing to do in life. Living in fast, technically advanced 21 century makes it more hard to do it, We can't abandon our responsibilities to become a hermit or monk meditating in the lonely Himalayas.
To make my point more easy and clear to understand:
Remember this famous "The Pit" scene from The Dark Knight Rises?
Yes, conquering our mind is almost like this. It looks so much easy but we fail again and again in the process of climbing this metaphorical pit of external things, which are disturbing our mind.
But anyone with dedication, deliberate practice and faith can climb this pit.
once, you have climb out of this pit no external things will affect your mind and life. You'll help other people to make the climb!
Quotes from Bhagavad Gita:
He whose mind is unaffected by misery or pleasure and is free from all bonds and attachments, fear and anger, is man, of steady wisdom and decisive intellect.
दुःखों की प्राप्ति होने पर जिसके मन में उद्बेग नहीं होता, सुखों की प्राप्ति में जो सर्वथा नि:स्पृह है तथा जिसके राग, भय और क्रोध नष्ट हो गये हैं, ऐसा मुनि स्थिर बुद्भि कहा जाता है ।। ५६ ।। (chapter 2, shloka 56)
So that is it. Toughest but most valuable thing to do in life.