In life, seems like there will always be someone who will not hesitate to tell me how they believe me to be wrong/mistaken/misguided/blind and/or foolish in my ideas, practises, beliefs, habits. I once was just this sort of person, who felt free to offer my opinions to all who would stand still long enough to listen. I justified unkindness as "just my opinion."
Al-Anon has taught me to use the acronym THINK:
Is it Timely? Is this something that shouldn't be brought up in the present situation, ie, in front of a group of people, but dealt with at a later date, when the two of us are alone together?
Is it Honest? Self-explanatory - am I being truthful?
Is it Intolerant? Am I being judgemental?
Is it Necessary? Can I use this opportunity to practise letting go?
Is it Kind? What is my motive for saying what I'm about to say? Will it hurt someone? Then don't say it.
I can't control how another person behaves, but I don't have to substitute their judgement for my own. If I know that I'm doing a good job, that's enough. I don't have to take it personally when someone is critical of me, because I know from my own experience of being that same critical person, that it arose from deep insecurities.
I've learned to use some noncommittal phrases:
"Oh, yeah." (said not in a challenging tone, but with a sort of mild disinterest.)
"Isn't that something!"
"You could be right." This last is hugely powerful in its ability to disarm someone who is riled up about whatever it is. It validates their opinion or attitude without agreeing to it, or submitting to it.
There's a critic in any crowd. I pray for serenity in the face of criticism, and the ability to learn from the constructive variety.