FOMO, fear of missing out, is the pull to get in because price is moving and you are not in it. The entry comes late, after the move you wanted to catch, often without the setup's trigger and with the stop placed wherever it fits.
Why FOMO entries lose
A late entry changes the trade's geometry. The logical stop is still at the same structure, but the entry is now further from it:
The same idea went from a 3R opportunity to a 1R one, and to keep the same dollar risk the size has to be halved. Most traders keep the size, which doubles the risk. Price also tends to pull back after a sharp move, so FOMO entries often see their worst point right after entry.
A rule against it
If price has moved more than a set distance from the trigger (for example 0.5R), the trade is gone. Wait for the next setup. Writing this into the pre-trade checklist makes it a yes-or-no question instead of a feeling.
Measuring the cost
Tag every late or unplanned entry as FOMO, then compare its expectancy in R with your clean trades. The difference, multiplied by how often it happens, is the yearly price of the habit. See mistake cost.
In SetupBook
"FOMO entry" is a built-in mistake tag and "FOMO" is one of the emotion labels. The dashboard's mistake card shows the money lost on FOMO entries, and Analytics shows their count, net result and win rate next to trades with no mistake.