FOMO is a fear of regret, not a view on the market. A price runs, you were not in it, and the discomfort of watching pushes you to enter at a worse price, with a wider stop or none, and without the setup you would normally require.
The cost is usually structural rather than dramatic. Late entries have a poorer ratio of reward to risk because part of the move has already happened, and the stop has to sit further away or goes unplaced. A series of such trades lowers expectancy even when some of them win.
Practical defences are mostly about turning the urge into a question with a written answer. Does this meet the entry criteria in my plan? Where is the invalidation level, and what is the risk in R at that level from here? If the answer is no or the risk is too wide, the trade is skipped and the skip is logged like any other decision.
There will always be another move. A journal that records skipped trades, along with what happened afterwards, tends to show that the opportunities you missed were fewer and less clean than they looked on the screen.