How should one handle criticism?

If the criticism is superficial, then let it be. It is one of the Navarasas (the nine emotions, which are, love, laughter, compassion, anger, courage, terror, disgust, surprise, and peace).
If you want something bad to happen to another from the core of your heart, from the depth of your being, then you are harming only yourself. The person being criticized won’t be affected. But that doesn’t mean you should be superficially positive, and never point out mistakes.
If an apple is bad, saying the apple is good is insincere. If someone asks you, ‘How was the food’, and you say it was very good, but you couldn’t eat it then that is not honesty. Where there is a problem, it must be pointed out, but without any hatred in your heart, otherwise the feeling will harm only you.
If somebody else does it, then let it be. What can you do? It is their rajoguna and tamoguna which makes such negative tendencies arise in them. But if you become negative towards them because of that, they you will also be in the same basket as them.
The three gunas, rajoguna, tamoguna and sattoguna are an integral part of nature. You just be a witness to their play and display. And if you want to explain it to somebody then do it because you care for them, do it with compassion. Tell them that "You have spent two hours complaining about somebody today, this will severely affect your creative or positive energy. The more you complain the more energy you will lose."
Tell them to complain about somebody for an hour and observe how much energy they have lost. When people start complaining about others they forget the time because they find some form of enjoyment in it. Like a patient enjoys talking about his ailments. If he has ten visitors, he’ll tell all ten people the same thing, ‘All morning I have been having leg pain. I could not sleep all night because of the pain’.
All the crying and drama is repeated over and over again. He should do something else instead of harping on and on about it. Write a poem maybe, or do some other creative thing. Then he can be happy and also forget his ailment.
Anything that is repeated, gets deeper into the consciousness. That is why mantras are repeated again and again. So don’t let complaints get deeper into yourself, instead do some japa (meditative repetition of a mantra or name of a divine power).

Effect of mantras

The chanting of mantras creates an environment of positivity and sacredness. Mantras have an impact at the subtle level also. Our physical body is the gross body, while our subtle body is 1.5 times larger than the gross body. And the Karana Sharira or Causal body is a 1000 times finer and subtler than even the subtle body itself. So we have three kinds of bodies. When the subtle body constricts and becomes smaller than the gross body, then a person becomes restless and miserable. When the subtle body diminishes even further, then a person loses his mental balance and feels like committing suicide. This is the root cause behind suicides.
By the practice of chanting mantras, pranayama and meditation the subtle body grows larger in size and also gets strengthened. When the subtle body becomes strong then one appears normal (physically). And when the subtle body expands much more than its normal limits then one feels that he is soaked in bliss. And as the subtle body expands further, it becomes finer and finer and merges or becomes one with the Causal body – that is when one gets Moksha (liberation or salvation).