Assuming that each one of us has a good side and an evil side, the darkness of mind can be twofold: One - the evil side (dark side) of us, and Two - darkness of ignorance. I am further assuming that you are referring to the darkness of ignorance and not the evil side.
There is a phrase in Sanskrit - Agnyanaandhakaram - meaning the darkness of ignorance. In Bhagavad Gita, this darkness is termed as darker than the darkest substance in the universe.
The darkness of mind (that of ignorance) is the hardest kind of darkness to remove. Why? The only light that can distance this darkness is the Light of Knowledge. The Light of Knowledge, again, is twofold. One, the knowledge itself, and second is the wisdom coming from the deeper understanding of such knowledge. "When intelligence matures, and lodges securely in mind, it becomes wisdom" (C. Rajagopalacharya).
Why is it so difficult to bring the Light of Knowledge into the mind? What is the knowledge we are talking about?
To bring the light of knowledge takes work - a lot of work. Knowledge, here, is the knowledge of the physical self (I, me, etc., and Jeevatma in spiritual terms), and the knowledge of Self (the Supreme Soul or Paramatma in spiritual terms). To bring in the true knowledge of self (me, I), it takes knowing about ourselves, questioning ourselves, knowing the good side and bad side of ourselves, admitting to ourselves our strengths, and more importantly, our weaknesses and shortcomings. Beyond that, it also entails making a conscious effort to expand our strengths and good side, and marginalizing our weaknesses and bad side.
While most of us admit that everyone has good and bad in them, as a natural human tendency, we tend to first see only the bad side in others and only the good side in ourselves. Seeing (and admitting to ourselves) the bad side in us and making a concrete effort to change the bad side in us takes real (moral) character and tremendous inner strength. Once we possess that strength and gain the knowledge of ourselves, we get halfway across in bringing the light of knowledge. If you are not spiritual at all, then you are all the way there. If you are spiritual, then the remaining half is to gain the knowledge and understanding of the Supreme Self.
Again, not gaining the knowledge of the Supreme Self is not the end of the world (and does not mean you are any less of a human being). If you just attempt to gain that part of the knowledge, you are better than most of us. As is said in the Bhagavad Gita (and adapted to the current world population numbers), "Among millions of people, somewhere in some corner, one person attempts to know the Supreme Self. Among those who attempt to know the Supreme Self, only a handful is destined to learn the truth about the Supreme Self and gain complete knowledge". Therefore, the fact that you even attempted to know puts you in a different plane (making you one in a million). Even if you gain a tiny bit of such knowledge, you are better off than most of us. If you gained full knowledge, then there would be a shrine built for you and you would be named Buddha (or someone similar).
Hope that explains why it is difficult to handle the darkness of mind. The simple answer is, it is not easy to bring the light of knowledge that distances/removes the darkness of mind.
Thanks,
-Som G