What are the main lessons we can learn from the Mahabharata?

 Honestly, a person could spend their whole life studying the Mahabharata and easily keep on finding new lessons to learn from it. And the main ones may vary from person to person. I think some of the main ones are the key philosophical foundations of Sanatana Dharma:

• Life comes in repetitive cycles of ups and downs: happiness-sadness, satisfaction-dissatisfaction, etc. We can see this clearly in the lives of the Pandavas and indeed in almost all the characters in the Mahabharata. The Mahabharata encourages self-awareness and management of inevitable struggles rather than fatalistic acceptance.
• At its base, the Mahabharata deals with actions and their consequences. Very few of the Mahabharata’s story beats are “random” events; most of them proceed as very direct consequences of the actions of the principle characters. The Mahabharata emphasizes that thoughtful responses tend to have far better outcomes than impulsive reactions.
• The Mahabharata teaches pluralism, compassion, ways to alleviate suffering, and methods for self-discovery and self-management.
• The Mahabharata fully teaches how to attain Moksha - how to manage or free ourselves from the repetitive cycles of dissatisfaction or sorrows that we are put into, or that we put ourselves into, thereby attaining a blissful state of existence. Indeed, to attain Moksha one does not need any source of knowledge beyond the Mahabharata; it contains all the necessary knowledge.
• Above all, the Mahabharata is an extensive treatise on Dharma, dealing with cosmic balance and disruptions to it. The Mahabharata beautifully teaches all the essentials of the spiritual tradition of Sanatana Dharma; it teaches the importance of fulfilling our individual responsibilities (sva-dharma), balancing them with our family/community values (kula dharma), and how to fulfill one’s responsibilities in times of upheaval.
• From the Mahabharata we can learn how to perform Sadhana - disciplined spiritual practice, either part-time as a part of normal life (pravṛtti), or as a full-time practitioner (nivṛtti) who has dedicated their entire life to the pursuit of liberation.
• In particular the Mahabharata teaches dhyana, meditation, practiced through abstract focus, devotion, or other methods, leading to deeper consciousness.
• From the Mahabharata we can learn the real nature of the Atman, the individual essence or self - that it is distinct from body and mind, eternal, and interconnected with others and with the cosmos.
• The Mahabharata teaches the nature of Brahman, the ultimate reality, the totality beyond all dualities; and Ishvara, the Supreme Being, as Vishnu, Shiva, and Durga.
• The Mahabharata teaches the importance of dialogue that sustains diversity of thought and respectful debate, and how extreme the negative consequences can be when this open attitude breaks down.
• The Mahabharata embraces a pluralistic outlook. Different paths are seen as valid for different people, and it generally avoids demonizing alternative practices, but rather, simply explores their natural consequences.