Well, to be honest- the main life lesson from Hinduism for me is to do your duty to the fullest ie- your Karma. Fulfill your duty of gaining knowledge and education, duty to serve your family, duties as a son, duties of a husband, duties to your people etc etc.
This is another misconception. Karma is not duty, Karma means Action.
Dharma is duty. Rta is the universal rule that governs lives, like Laws of physics.
One can act responsibly or act irresponsibly.
One who acts responsibly is called doing their Karma as per Dharma i.e. Doing Action in line with their duties.
One who acts irresponsibly is called "Bad karma" and practicing "A-Dharma" i.e Bad Action that is in contravention to their duties.
This is what is condensed in the Hitopadesha as,
"Ayam nijaha paro veti ganana laghuchetasam,
udaaracharitanaam tu Vasudaiva Kutumbakam"
Translated as ,
"He is my kin and others are not is for the Narrow Minded,
For the broad minded , the whole world is his family".
more & more you read or observe the greater will be your confusion. Fact is that there is no single authoritative source from which you can define hinduism. More over it will be pretty hard to relate to practical Hinduism. It will be like theoretical physics.
That is very accurate.
Which is why if someone really wants to understand the distilled essence of Hinduism, the best book is the "Bhagwat Gita".