I've had some tough beats at the SnG's lately...made it to the bubble twice in a row only to be beat by someone calling my bets with worse hands and drawing out on me. Sometimes you wonder...
...but, when you start to wonder, I agree with JamaicanKid. Just walk away, go do something else, and believe it or not, you will feel a whole lot better.
Remember, this is the short-term. If you took those exact same scenarios and re-played them over and over, you'd come out profitable most of the time (only if you feel that it was the very best play you could have made).
Play for the long-term. You're not alone...it took me about a whole year of playing poker to finally be able to control my emotions, especially when the bad beats come. It happens to all of us.