In making notes on games, I've probably explained lots of chess mistakes, and why they were mistakes. I might say, this Black move is a mistake, because White now plays A, and this works because if B then C and if D then E. (Or, White should have played A, etc.)
But there is another side to each mistake, which I can't tell anything about, but which perhaps you can, and you should try. A mistake in a chess move is also a mistake in thinking.
So, on reflection, was your mistake any of these?
1. You didn't look for the threat - it didn't seem that sort of position.
2. You looked for threats but you didn't notice it
3. You saw the threat but you hoped it didn't win
4. You saw the threat but you hoped your opponent didn't play it
5. You misunderstood the position - you thought something was important that was not (or dismissed something that was really important)
6. You saw the threat and missed something in the analysis (really, this is going to be some variety of the first five)
So, as well as reflecting on the mechanics of a chess game and where it might have been improved, also have a think about the workings of your mind.
If you can catch yourself making a mistake in thinking, that is helpful. If you catch yourself making the same mistake fairly often, then that's a habit you need to get out of!