Most people pose the question this way: Are you consistently profitable? I am profitable, but not consistently. However, I don't need to make money every month. I just need to make money overall, and save some of it during the up months for use during the down months. Simple budgeting.
You need a plan you believe in. If you don't believe in it, you won't follow it. As Faith says, backtesting is what we do because there are really no alternatives. Backtest it till you believe in it. If you cannot believe in it, you must find a different method, and backtest that one. Once you have a method, you must trade at a level you can sustain through thick and thin. If you have a losing day, week, month, or whatever the time period of interest is, you must be able to go back and apply the same method again, using the same percentage of your account. If you are psychologically unable to do that, you are either trading too large a proportion of your account or you do not believe in your method.