You can't avoid "falling into the wrong trade" (unless you can see the future, but then what are you still doing among us mortals?) but you can avoid staying in the wrong trade. The secret is known as stop loss, and you decide where it goes before you enter the trade. How far you put it should be inversely related to your position size, which in turn is a function of your risk appetite (a.k.a. sanity) and is computed from the implied volatility of the market.