One cause of unhappiness is ruminating: replaying an unpleasant thought over and over again. This could be something that happened in the past — say, you’re repeatedly thinking over something annoying that happened to you today.
Another cause could be repetitively imagining something happening in the future — say, a difficult conversation you need to have with someone, or even an argument you are secretly imagining having with someone.
I’ve found in my own life that rumination can be both exhausting and pointless. If something bad happens, it’s helpful to acknowledge it, accept it and move on. But dwelling on it excessively isn’t helpful. In fact it makes the problem worse.
If you catch yourself ruminating on something, then what? I’ve found just acknowledging that I’m ruminating on something isn’t enough. It’s not enough to break me out of the cycle. It just keeps me in the over-thinking mode.
What does seem to help is thinking about things on the physical level. For example, if I’m ruminating while washing dishes, I’ll start to think about the sound of the water and the weight of the pan that I’m scrubbing. Basically, less thinking, more experiencing.
This usually does the trick.