it seems i can only play my best chess for a couple of hours . After that it seems im mentaly drained. I went to a friends house to play chess and a year or so ago i wasnt close to beating him. He plays alot but last night i won the first 2 out of 3 games and feel like i should have quit but i just kept on playing and i lost the fourth game , then next , ended up losing the rest . we played for 6 hours.  and the more i sat there  the worse my game got. Ive never played that long straight. is it normal to only be able to play your best for a short time ?

I've never had that problem, but in the few OTB tournaments I've entered I've seen others' play get worse after a long day.  Probably the more tournaments you play in, the less it will be a problem.  You should also make sure you eat and drink enough if you're playing for a while...it can be easy to forget.

same here, I played OTB for 3.5 hours and was drained. won the 1st game, but it was over 2 hours long and 65 moves against a good player but lost the next one without any effort before barely drawing the 3rd game.

Im used to corr. where I can take breaks(and move the pieces to my calculating super easy). the solution is to build up tactically where youre setting up the problems on a real board and going longer and longer each time.. .while still not resting on the longer calculations. also the de la maza drills help a lot. hopefully I can get back into OTB shape in a few months by april or so

yeah lol.. I could not imagine playing for more than 4 hours.

I heard that Nigel Short game from a few weeks ago went 8 hours (!?)