There are many reasons for this but the number one reason is trading your P&L instead of trading the trade. The only way to hold on to good trades longer is believing in your idea (but being flexible to change if it is not following through).
The only way to take it to the next level in your career and not focus on the small fluctuations is to do a better job of building and “argument” for a good trade just like a lawyer builds a case in court.
The better the argument for your idea the more confidence you will have in the position.
BTW this is also how you will keep yourself from over trading, the weaker the argument the less you will trade.
**This of course assumes you know what a good trade looks like! 
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