You know you are capable of playing great poker, but you continuously lose your bankroll playing online poker. How do you get over this hurdle? What is the secret? There are two important factors.
Factor 1: Don't play on tilt
Winning poker is not about focusing on how much you've won or lost. It's how you handle losing. How often do you get bad beat, then jump right back in and lose more and more? How often do you make a big win, only to lose it all over the course of days or weeks?
So to overcome factor 1: If you get a bad beat or get on a losing streak, force yourself to take a break before playing again. How long or what this is depends on what it takes to get your mind free of the past. Maybe it's a nap. Maybe it's a walk around the house. Maybe it's a jog around the block. The important element is that you are not playing the next game with the predisposition of steaming. Get over it first, then continue playing.
Factor 2: Bankroll management
Losing is a fact of the game, it will happen. To stay alive, you must manage your bankroll. This sounds easy, but it can take a considerable amount of discipline. If you don't want to buy in again, you *must* manage your bankroll, no matter how good you are. A rule of thumb for sit-and-goes and MTTs: Take the buy-in multiplied by # of players. If that amount is greater than your bankroll, move down.
If you get on a losing streak, just stick to the plan. Take a break, and move down in stakes until you can recover.
Follow these rules, and the only way you can lose is by playing bad poker. Hopefully you've gotten past that part :) Good Luck!