Mental stamina is intimately linked with physical stamina.
You need to be physically fit to operate well mentally for long periods of time like that.
Also energy is important, lots of carbohydrates and starches for long term energy, there's no point starting a game stuffing sugar down your throat, its too short term an energy boost, and will leave you even more tired after a long while.
I have one problem!i am a 1360 fide-rated player!when i play with a player with rating 1600+ i play well but after 2.5-3.0 hours game in an equal or winning position i make a mistake and loose the game!what can i do to stop doing this?