These are some point’s I have learned and also heard from experts.
1. Having no plan or edge over the market. You need to have a system that has a positive outlook and it’s repeating.
2. Thinking it’s about the ‘one’ trade. Thinking about one trade is a warning that you are risking too much, because no one trade should risk your existence. And it’s impossible to find systems that are right always. You just need to be right more times than wrong. The result is the outcome of many trades over given period(month/year)
3. Wrong position sizing. Risking too much or too little for individual trade. You should calculate the risk for your account and make positions that don’t risk your existence, but they still need to be big enough to make a positive impact too.
4. Style drift. Beginner traders style drift, they may take a day trade, but after successful start they want to extend it and drift to swing, or take wait for swing for weeks, and then just take profits within the same day. Usually problem is that they don’t have a written system/verified edge and mental issues make them change the style. You can have multiple styles, but decide the style you are using before making the trade and write it down.
5. Not understanding stop loss usage and it’s effect to accuracy. Cutting losses short is very important, but using stop loss makes your accuracy lower. It’s not possible to use very small stop loss, because it’s going to hit always because marked movement. You have to limit big mistakes by selling losers, but if trade is solid, company has not changed and your entry is good. You need to give it room to breath a little bit.
6. Not having daily / weekly stop loss. Most expert traders have these. They limit the death spiral and revenge trading, where trader start to win back the losses in aggressive manner. Usually just generating more losses.
7. Not keeping Records and statistics. To improve your skill, you need to have records. Without these it’s very hard to even verify you have working edge over the marked. And what trades should be eliminated to improve the returns. Google docs spreadsheet is a good way, enter stock name, entry price, exit price, stop loss level of method, profit/loss, used method(important if many systems in use, you need to compare your day traders to your swing trades), reason for entry, reason for exit, and lastly feelings(to understand if the issue was a mental state).
8. After loosing, most people are too fast to take small profits on winners, Good idea is to use rules, in swing trades you can use days(don’t sell before 6 days). Or sell half of the position to satisfy the greed and let the other half room to run, just raise the stop loss slowly. <-common expert trader method.
9. Unrealistic market expectations, market doesn't understand your needs, you are trading against experienced professional traders, and winning consistently is very hard even for experts that work full time. Fighting for profit in that market is very hard. getting away consistently even with small profits is a real skill.
10. Not having written down rules, playbook. You need written down rules that you must obey while trading. This limits the mental issues while under stress. And you need playbook, this is trade methods, with rules, what to look for, how to take it, how to size the position, how to manage possible loss, when to exit if it goes south, or if it wins, and even if it's not going anywhere. And after testing these methods you can fine tune the method, improve it.
11. spending time while trading, you don't need to take trades, you don't have to have positions. If you don't see good trades, don't take any, Most likely you can occasionally win but without real focus you are just standings still and causing commission fees.
12. Following other people trades, This is beginner mistake, find your own setups, and know what makes your edge.
13. Trading unknown instruments, You have to know what you are trading, in most styles you need to know the stock individually, how many times it's generating certain trade in a year, what are ways to find out the position.
Some of these are my own findings and some these are taken from various sources, one of the best resource is 'chat with traders' youtube channel. Check it out!