Not cutting losses in losing positions
Watching profits evaporate into losses and not acting!
Most new traders when they have a view on a stock, option, futures or FOREX position will usually have an idea where they feel the major support/resistance areas are to cut losses and take profits. However, just like gambling in casinos, they tend to reevaluate and change their minds and push their luck at the last minute.
This usually happens because when it comes to trading, if you don't stick to the "original plan" when entering a position and that market reaches that level, you don't know how to react.
The problem lies in being conflicted on what type of trader one is. If someone is a swing trader where they look to buy/sell on breakouts until the market reaches a certain support/resistance level than stick to that model. But, a lot of traders then turn into macro traders which are defined as someone who holds positions for more than several weeks or months and this leads to a deviation in short-term trading vs. long-term trading.
A perfect example would be if you believe for example AMZN with its parabolic state will come down slightly to say support at $1550 because you see overall stock indexes are starting to show weakness, then you need to make sure to get out of a cheap OTM put when it starts going your way to that level. Usually what happens though in options trading is the price starts going your way and you see the premiums explode 5x...10x..15x and then your greedy side comes in and say's "hmm, maybe now it can come down even more" This flawed thinking tends to see your premiums evaporate or slowly come down as volatility starts to dry up because that stock is sitting on major support now.
Now you don't have to close everything out at once if you have multiple options contracts on if you feel the stock will run lower, but it's generally a good idea to take significant profits off the table to lock in profits. Most don't do this. They will watch the price level hit that they predicted and think it will run more. What tends to happen is the stock stays bouncing back and forth at that major support and premiums start to come down, which makes them panic and flatten their position making less money than they should have.
When putting in a trade, have a DEFINED entry and exit point established to take profits and losses and don't change your mind once the market moves at either level. You're not a hedge fund making these big macro decisions on how the world will shape. The goals in trading small accounts are to take profits and redeploy as quickly as possible in growing the account.