Once a position is open, new information arrives continuously and almost all of it is noise. Second-guessing treats each tick as a reason to reconsider, and reconsideration under pressure tends to produce the exit that feels best rather than the one the plan specified.
It is also self-reinforcing. Each mid-trade change that happens to work teaches you to do it again, and the record then contains no clean test of the strategy at all.
Decide when reconsideration is legitimate and write it down: a defined structural change, a scheduled event, an invalidation. Everything else waits for the review. See plan-abandonment.
Related: plan-abandonment, analysis-paralysis, p-and-l-watching, hesitation