Sometimes it’s a recent event, like how a meeting or a conversation went; sometimes it’s an event in the past, like the breakup of a business partnership (or a personal relationship), or being let go from a job.
When you start to notice how you talk to yourself about it, you will likely notice unconstructive thoughts, such as “you should have done it differently!”
If your reaction is one in which you blame the other person when you replay the situation in your mind, it may very well be the case that the other person was doing the best they could but didn’t live up to your expectation or your preference for how you would have liked the situation to go. Maybe, objectively speaking, what the other person did was inappropriate. However, even though it seems like you are angry at the other person, that is often not what is REALLY causing you to stay hooked on thinking about it.
Here is the REAL reason why you are obsessing about it: The situation unfolded the way it did. That’s now a fact. But when you explain to yourself why it happened that way, you have made the situation to be a confirmation of a long held belief you have about yourself (e.g. "I am not good enough"; "I’ll always be a 'B+' kind of player"; "I’m a loser”, etc.)
To start moving forward, what you want to do is start to trace “what it means about YOU” that the situation happened the way it did. Write down on a piece of paper the explanation(s) you tell yourself for why the situation happened this way. With each answer you give, dig a little deeper to answer the question “and what does that mean about me?” This analysis will lead you to the root of what is making you hold on to the situation. You want to see if you can come up with a personalized meaning that confirms your deepest fear or doubt about yourself.
For example if you are still upset about a business partnership breaking up, see if you can identify the deepest concern you have about why it didn’t work out. If your answer is “I think ultimately the business partnership broke up because I wasn’t smart enough,” notice how you have condemned yourself. It’s kind of hard to move forward when you think you have confirmed that you don’t have what it takes to succeed. That’s why you obsess about it – you keep thinking about it to make it different in your mind, or to debate the merits of whether that belief about yourself is true or not.