Commentary on Meditations: B3:4

Do not waste the remaining part of your life in thoughts about other people, when you are not thinking with reference to some aspect of the common good. Why deprive yourself of the time for some other task? I mean, thinking about what so-and-so is doing, and why, what he is saying or contemplating or plotting, and all that line of thought, makes you stray from the close watch on your own directing mind. No, in the sequence of your thoughts you must avoid all that is casual or aimless, and most particularly anything prying or malicious. Train yourself to think only those thoughts such that in answer to the sudden question 'What is in your mind now?' you could say with immediate frankness whatever it is, this or that: and so your answer can give direct evidence that all your thoughts are straightforward and kindly, the thoughts of a social being who has no regard for the fancies of pleasure or wider indulgence, for rivalry, malice, suspicion, or anything else that one would blush to admit was in one's mind.

A man such as this, if he postpones no longer his ready place among the best, is in some way a priest and minister of the gods. He responds to the divinity seated within him, and this renders the man unsullied by pleasures, unscathed by any pain, untouched by any wrong, unconscious of any wickedness; a wrestler for the greatest prize of all, to avoid being thrown by any passion; dyed to the core with justice; embracing with his whole heart all the experience allotted to him; rarely, and only when there is great need for the common good, wondering what others may be saying or doing or thinking. He has only his own work to bring to fulfilment, and only his own fated allocation from the Whole to claim his constant attention.

As for his work, he makes it excellent: as for his lot, he is convinced it is good. And each person's appointed lot is both his fellow-passenger and his driver.  He bears in mind too the kinship of all rational beings, and that caring for all men is in accordance with man's nature: but that nevertheless he should not hold to the opinions of all, but only of those who live their lives in agreement with nature. He will constantly remind himself what sort of people they are who do not lead such lives - what they are like both at home and abroad, by night and by day, they and the polluting company they keep. So he disregards even the praise of such men these are people who are not even satisfied with themselves.

This passage has it all.  Let me summarize with tweet-like thoughts:

Don't worry about what others are thinking or saying or doing.

Your thoughts should be centered around what is in your control.

What is in your control are your actions and your attitude.

Your actions should be social and for the benefit of the community.

Lead a life of grit!  Don't give in to pleasure or pain; be morally good.

Love your fate - what the universe/God has dealt you.

Choose your friends wisely; you become like the company you keep.  If you wallow in the mud with the pigs, you will smell like a pig.  If you walk in the rose garden, you will smell like roses.