Judging the risk is half of the work, that's why most people fail at that level. You have to calculate the risk/reward ratio very carefully and if you do a good job then you are going to get what you want, but if you do a terrible job then you are not going to get what you need. Most of the time people make a mistake in that part and because they make a mistake, they end up with bad results as well.
For example, they risk too much for too little reward potential, but they do not calculate well, either the risk part or reward part, they either think a high risk is actually low, and make a mistake, or they think reward could be higher than what it will be. That is why they end up with bad results, and they need to do this better if they want to be a good trader.
Not learning from mistakes is another one as well. I understand that risk/reward ratio calculation and failing to calculate that properly is one of the biggest ones, but after they make a mistake and buy something that has high risk and low reward, then they should learn to do better.
Unfortunately there are so many who make that mistake and then keep doing the same mistake. A great example would be memecoin people, they invest, and they lose it all, and then they invest again, for what reason? Because they ended up with seeing some influencers or some stranger online make some huge returns on some silly shitcoin. Just because one in a million people got lucky, doesn't mean you should risk your money too. So learning from mistakes is a big one too.