def not alone, i too play 5/10 6max usually and occasionally jump to the 10/20 6max. i have decent numbers playing the 5/10 but when i get to the 10/20 the variance and swings are just too great. sure you can set aside 300 BB for the game, but when you drop 50-100 BB in a game you get rattled and your off your game, that is going to happen.
just keep on stabbing at it once you build a good enough BR and your day will come...