This is one of "22 virus definitions" (thought-mistakes that cause ineffectiveness and unnecessary negative emotions).
This is the same sort of mistake as overcertainty. When you make a guess, it helps to realize it is just a guess, especially when your guess is making you feel defeated, demoralized, or otherwise unhappy. As soon as you realize your negative guess was just a guess, it takes the power out of it and you stop feeling bad.
I know a man who is cynical about the president, the governors, and politicians in general. He is glum often because he thinks he knows what they’re all after. He's sure they’re only interested in money and power and they don’t care about “the people.”
In other words, he is guessing the motivations of politicians and then he becomes depressed and bitter because of his guess.
People do that not just about other people, but about future events. “It’s not going to work out,” you might hear someone say, and she believes her guess, so she doesn’t even try. She is guessing what’s going to happen in the future. She’s making a negative guess and feeling defeated by it.
Can you see how much differently it would be to not even not bother to guess but to merely acknowledge to yourself that you don’t know? If that’s the truth, if you really don’t know, then just admit it. Not knowing may make you feel an uncomfortable degree of curiosity or frustration, but it won’t make you feel demoralized or angry. It won’t make you want to give up on a goal.
See the complete list of definitions: The 22 Virus Definitions.