I think this is the biggest problem all players have. They all know pocker , they play their best for a period of time but after that the start doing -EV moves. So there is a way to deal with it.
Educate yourself to split you playing time into more sessions with big breaks after them. One, one and a half hour sometimes even less than this. You live the pocker site no matter how good you are doing. Go do something else , read, study, watch TV, play a game, sleep. Anything but not actually play. I have this problem myself and untill recent i did not found the best way for me to deal with it. I split my playing time in 1 hour sessions. I play more than this only when i play CC freerolls
. It does not matter if your winrate is small. At least you are winning. When you start playing bad you will quicly loose all your wins and more. Playing the same good pocker all the time makes you deal with variance. Playing good and than bad makes you a loosing player.