Most beginning traders fail with a real account after a success on a demo first and above all due to emotions, say, if you had a $100,000 demo and then you start to trade with $1000 you soon get disappointed with the sums you win and then there's temptation to put more and more on stake. The result is obvious.
Another side of the emotional component of trading demo is that you are tempted to wait in a losing position, and sometimes you can wait for it to turn profitable. But you cannot do it on regular basis on a real account which is, say, 10 to 100 times less than the demo one. So, if you trade a demo be sure to make the deposit similar to what you're going to trade with in real, or just enlarge your positions to preserve the amout of money at risk to the whole deposit sum ratio.
But all in all, emotions are what prevent a trader from becoming a success. If you have emotions it means that you do not have a profitable
system, backtested and robust. So trading without a system with real money inevitably leads to the loss of the whole deposit. You should start off with money management, then system building, then demo trading and only after that begin to work with real money. Otherwise the whole thing is not
trading, it's mere
gambling.
I can name at least two brokers at once which offer a lifetime demo which is
exactly the same if you trade small positions (say, up to 1 million) and they offer you
any size of demo deposit you like, so you can check your
robust and profitable system in conditions as close to real trading as my hands now to the keyboard. I mean Oanda (
http://fxtrade.oanda.com) and Forexite (
http://forexite.com).
As always, good luck.