Usually I manage the risk by contoling the sise of my position. On that losing trade, I suddenly got emotional and kept adding the position when the trade went against me. Technically, my bet is not really wrong. It's the big size of my position (relative to my asset in the account) that killed me. After I increased the size of my position, the trade went further against me. I found I may be wiped out so I liquidated all of my positions unconditionally.

Although we say we should not make any bets that we cannot afford, I had never understood that better before.