A common type of distorted thinking that people experience is the fixation on the negative things that happen. There could be a day full of positive experiences, but if a tiny negative experience happens, our brains can get fixated on it and it washes out all the positive things.
As a hypothetical example, let's say I knit a sweater and show it to 10 people. 9 of them say things like "Wow, that's so pretty!", "That looks amazing", "You are so talented, I could never do that." As a result, I'm feel pretty good and proud of what I made. But then 1 person says "I personally would have chosen a different color." The distorted thinking kicks in and my attitude shifts. Now all I can think about is how maybe I should have used a different color, that it was a waste of time to make that sweater, that everyone secretly hates but won't tell me, that I don't have the talent to pick the right color, why do I even bother, etc.
Intellectually, I may know that 9 people loved the sweater, and only 1 person said something that could barely be perceived as negative, but my brain is stuck. I can't get out of the cycle of thinking that I'm worthless.
This sort of distorted thinking is not necessarily an indication of mental illness. If left unaddressed, though, the feelings of worthlessness and self-hatred can escalate into something more serious, such as major depressive disorder. A therapist can help sort out these feelings and teach you strategies to have a healthier response to negative situations.