Fear . I think this is the problem most traders encounter. And my self is like that, the time when I first invested I lost a lot because of this. And now I have improved it.
We all know this thing that trading is not that much easy thing to do most of the people think of it to be and that the most common mistake which they make because of this misconception is that they jump into trading without even knowing anything about any of the crypto coin or anything related to that of trading. Other than this, loss of control over their emotions is also a main reason which result in loss of money for them.