The term weak longs is often thrown around quite loosely. Is a weak long someone that immediately closes out their positions? Does this trader often second guess their position the second after they put the trade on? Is this trader constantly looking back to see that a trade he closed out for a minor loss ended up running much higher? A weak long would answer yes to all of these questions.
As an active trader I would say that the majority of weak longs are made not born. Traders come into the market to make money. That is the bottom-line. What tends to happen over time is that new traders will often take the early lumps associated with starting out in the game. These "lumps" will often cause traders to go back into their trading shell. Traders will go through cold streaks, much like an athlete, but you as a trader can not allow your cold streak to turn into bad habits. The stock market is about odds, so you have to assume that "x" percentage of your trades will not work out. If you find yourself constantly staring at your stock 30 seconds after you entered the trade, and find your hand shaking, you have now joined the weak long club.
Below are a some solutions that can improve your trading if you find yourself unable to hold positions: