Emotions are the key. I also noticed that my demo accounts always prosper, but the live account blows. I realized the BIGGEST difference was in my behavior. When I had a trade in the demo, I would leave it to either hit stop loss or take profit. But on the live, if i hit stop loss on one trade, I start panicking, and the minute I get a little positive on another trade, I immediately close it. So i would have a lot of trades in profit, a few pips each, but one loss would wipe out all my profits.
I have since learnt to trust my analyses and let it ride, using proper risk management. And also I found a way to reduce the balance in my demo account to a value that I am likely to fund my live account with, something like USD50 or 100, and not the crazy USD10000, that is found in many demo account, it is unrealistic.
I'm happy to share it with you if you want. It only works on demo account and so you don't run the risk of running it on a live account.