Bitter behavior is so common and deeply destructive that some psychiatrists are urging it be identified as a mental illness under the name post-traumatic embitterment disorder.
According to the article in the Los Angeles Times:
Embittered people are typically good people who have worked hard at something important, such as a job, relationship or activity, Linden says. When something unexpectedly awful happens -- they don't get the promotion, their spouse files for divorce or they fail to make the Olympic team -- a profound sense of injustice overtakes them. Instead of dealing with the loss with the help of family and friends, they cannot let go of the feeling of being victimized. Almost immediately after the traumatic event, they become angry, pessimistic, aggressive, hopeless haters.
"Embitterment is a violation of basic beliefs," Linden says. "It causes a very severe emotional reaction. . . . We are always coping with negative life events. It's the reaction that varies."
There are only a handful of studies on the condition, but psychiatrists at the meeting agreed that much more research is needed on identifying and helping these people. One estimate is that 1% to 2% of the population is embittered, says Linden, who has published several studies on the condition.
"These people usually don't come to treatment because 'the world has to change, not me,' " Linden says. "They are almost treatment resistant. . . . Revenge is not a treatment."
Bitterness isn't a particularly pleasant or productive state of mind, but I don't think it's a mental illness either. It's just a less pleasant side of normal. Under some circumstances it is the world which should to change, not people who quite reasonably hacked off with the state it's in.
The people who are most effective in changing the world do transcend or overcome bitterness, but they don't lose sight of the fact that things need to change. And a very good thing too - we need the awkward sods who refuse to see unfairness and injustice as normal and refuse to accept that they are abnormal for refusing to turn a blind eye and go along with the herd. That's what people can do when at their best, but for most ordinary people, most of the time, being treated badly leads to resentment - it ain't big, it ain't pretty, but it ain't a mental illness either. As they say, just because you're paranoid doesn't mean they're not out to get you...
If I was looking for a way out of bitterness, I think I'd turn to a philosopher rather than a Californian shrink with a new syndrome to sell. I rather like the Japanese proverb which goes:
If you sit by the river long enough, you will see the body of your enemy float by.
I'm not entirely sure what it means, but my interpretation is that you're more likely to win life's battles if you can step back and attain a sense of proportion and inner calm rather than hating the person who's done you wrong which will hurt you more than it hurts your enemy.
Easier said than done, but a state of mind worth striving for and, when attained, a hell of a lot cheaper than therapy.