We all have our personal demons, and those come out if you trade.
My degree in trading has cost me a little bit over $55,000 (plus gains) in 4 years, and most of it was lost in a single trade. Every trade lost should be a lesson up to a point, but if you don't identify the core problem, you'll keep making the same mistake.
I've been doing currencies (focusing on the EURUSD). My broker at that time accepted opposite position, so I could both buy and sell at the same time. It gave me a thuarped sense of "hedging". The crisis in Greece was in its infancy, and the thing was ready to break through 1.4000. Using fundamentals (pointless for short-term), I sold. Account was at $72,000 (had made $22,000 in only a few weeks).
Usual mistake people make is averaging down. The position started going against me, so all I saw was potential higher profits once it went my way. This only led to the position getting bigger and bigger (higher loss per pip), and having to hedge with bigger and bigger buy trades. A single pip was worth $300 at one point.
Another mistake is guessing what the top/bottom is in a price swing. Just because you think that it has gone up enough, or a certain level has been a resistance at one point, doesn't mean shit. You may think who is stupid enough to buy past a certain price, but the market has no reason of being (it does, but that is manipulated so don't fight it).
Yet another mistake is setting a hard price target (unless your system calls for it). In my mind, I knew the Euro was going to 1.38 (of course eventually it went to 1.200), and I was set to make a cool $30,000 on the position. At one point I was down to $8,000 from $72,000 (no margin call due to the buy and sell trades open), so I started micromanaging the trade using the 1 minute chart. Closing the buy position when it was going down, opening the buy position when it was going up. I got back up to $45,000 and felt on top of the world. I knew for sure it would keep going down. NOPE, the market doesn't give a shit. A mistake in the way I set up a limit order, and overnight I woke up to having $18,000 only.
Finally, my own personal demon is I don't like to be wrong. Pride gets hurt, and you feel like the market personally knows what you margin call level is, and it's only acting that way to fuck you personally (except the market doesn't give a damn). I can win 5 trades in a row, but if I hold on to the loser because "I KNOW" that I will be right in the end, I get rammed. For the longest time my seldom sporadic losses always ended in margin calls, wiping out any profits accumulated and then some. That one trade could have ended up with me having $68,000 or more (at worst $45,000), and not $8,000.
Anyone can read about this all, but we all discover our own personal demons when we trade. No book or post will ever bring you face to face with that.