What's the most difficult emotional issue in trading?

Clearly it's the uncertainty of the trade. A trader is always confronted with a lack of confidence about the future. Even if a trader has a system of buying at support levels and selling at resistance, or buying or selling breakouts, there is always the uncertainty factor. No one really know whether support or resistance will hold

This uncertainty leads to a lack of confidence and a lack of confidence invokes the negative habit patterns of the trader. If a trader's weakness is the inability to pull the trigger, then the lack of confidence will always lead to the trader being tested in the area of pulling the trigger.  

How do we overcome doubt and build confidence? By finding a trading system with a high expectancy ratio or reliability factor. And how do we know if our sytem has a high reliability factor? Test it! Back test it. Paper trade it for a while and keep a record of your trades. Then put your system to test with real money using mini lots, until your confidence level is sufficient to increase your position size.

The key to good trading is confidence, and every trader will go through periods of waxing and waning confidence. Only trade when your confidence is high.