ADVICE with an OTB tournament I'm in.

Last night i played in my first over-the-board tournament round 1 and my next game is this afternoon. I need help with a few things.

So after playing as Black against the highest rated player in the tourney for 290 minutes i blundered just after i gained "the win" according to Fritz. I Felt mentally exhuasted, both having 5 minutes left on the clock but i think i blundered because i could feel numerous players around me watching my game because ours was one of the last to finish. I felt added pressure and i think this was the main reason for my mistake. How do you combat this?

Should i take breaks and just go to the next room for some water to break up the play?

This is your first. Don't be too hard on yourself. Get some rest the night before and drink plenty of water. You played the toughest now play the next.