I understand that your not good at trading hence the demo account, this is probably because you dont understand what it is your doing. If you took the time to research and learn before you throw more money away then it will make you a better trader and you might even be able to graduate to a real money account.
I've drawn a picture and put a link in to an explanation I gave a long time ago that no one has invalidated.
If you bought fx at the highs and sold later you would have made a loss. If you bought the index at a high you just have to hold on to make a profit. (Its worked for 100 years).
Sellers are just people who bought earlier at a lower price and are taking a profit.