Demo account is perceived as a kind of game. Even if you try to take it seriously, you don't understand real consequences and cause ot this you aren't getting nervous when trading demo account. On the other hand, when trading real account, you trader always has fear of loosing money. And that's the main difference. The only way to overcome this is to trade demo account only until you understand how trading works and then immedeatly move to trading with real money. It's okey that in the beginning traders lose money and that's the most fruitful period in terms of education and mastering stamina, but only those who overcome it, will become professionals and begin earning life trading.