Karna and Arjun were born to Kunti but ended up fighting for the opposite sides. Karna was cursed because of which his knowledge and experience of warfare didn't come to his rescue during the crucial fight with Arjun. He lost the war and was killed.
This situation applies to all of us as we're like Karna, we learn a lot during our lives, gain knowledge and experience. But at crucial moments we think and act on our instincts rather than out of awareness, as the depth of our awareness is below the required threshold. Krishna is fully aware of this and repeatedly explains the reality and truth from different angles in the Gita, so that the awareness sinks deeper and crosses the required threshold - the event horizon.
The Gita emphasizes that we have an inner self and an outer self, which are like two banks of a river. Usually we identify with the outer self, consisting of the physical body, our emotions, thoughts and the world around us. Krishna tells us to realise the truth and identify with our inner self that pervades all beings, is eternal and immutable. The enlightened one concludes, after reaching inner self (the other bank) that there is only one shore and the other shore is like the elusive snake in the rope-snake analogy.
The instruments of awareness include: Transcending Polarities (Dwandwa-ateeth); Transcending gunas (Guna-ateeth), Equanimity (Samatva), Being witness (Sakshi) than Kartha; and Independence of Karma (action) and Karma-phal (fruits of action).
It's better to read the Gita (especially Chapter 2) several times rather than read a hundred books, as each reading of the Gita brings out a different flavour and improved realization in us, lets the awareness about self sink in systematically and lets joy flow out.
Source - Daily World