I understand that there are two ways to make money from forex: Short-trading, and Long-trading, right?
Long-trading is when you buy a currency when it's cheap, and sell it off when it's high, right?
Now, the point where I was lost is the short-trading. It's selling a currency when it's expensive, and buy it when it's cheap, am I correct? If so, I'm kinda confused, because how can we sell something that I don't have?? (How can I sell USD before I even bought USD??).

I also have one more thing that keeps me confused. When I play long with EUR/USD for example, it means that I'm selling USD to get EUR right? I would get this USD from the 'margin' times my equity right? (I bought this USD using the balance in my trading account, correct?). And then, after this trade, I would have some amount of EUR right?
This is the point where I got lost.. I mean, okay I do have EUR 80,000 in my pocket (trading account) right now. So what? Let's say EUR/USD is hitting the bottom, and it would cause me to lose a lot of money, if I sell it right? So, what happened if I just leave it there? I don't sell it until it's getting back on its feet and touch the sky again.. Can anybody explain how can I lost more money than the one that I traded previously??