For example, if:
* You buy on the way down, and it keeps going down (should have waited), or
* You sell on the way up and it keeps going up (should have waited), or
* You buy on the way up and it keeps going up (you should have bought earlier), or
* You sell on the way down to stop losses and it turns around, or
* You don't buy at all (opportunity loss), or
* You don't sell at all and hold those losses, or...
You can always view whatever action you take (profitable or not) as poor.
The perfect is the enemy of the good. In this business no matter what you do you can beat yourself up over it. Only if you call the bottom and sell at the top can you really feel good about it, but even then you could be wrong on a different time frame.
That investor psychology issue (as well as profits) plays a big role in the attractiveness of momentum investing - buying on the way up and selling somewhere higher.
Not actionable information, but nevertheless it's good to remember that if one is so inclined, one must somehow escape any inherent negativity to survive in this field, as one can't optimize perfectly. Go for profits without perfection.