This worked for me when I was 2030 uscf and was attempting to climb my way to uscf master rating in 1 year. Psychologically I had 2 goals/methods:
1) Play every game based on the board, and try to play the best moves no matter how crazy or complicated they seemed (or how quiet and dry in some cases). Whether my opponent was 1600 or 2450 it shouldn't matter and I should play the same move regardless, since my focus wasn't to beat my opponent but to find the best moves to play. Playing guess the move on thousands of gm games, double checked with engine analysis, helped solidify this into a habit. Also telling myself again and again not to care about the result, but instead to try and have an entertaining game I can learn from.
2) I would always avoid looking at my opponents rating on the pairings, and would tell my friends to NEVER tell me the rating of who I'm playing against. Knowing this would only distract my attention away from the board and trying to find the best moves. If my opponent was magnus I would still want to aggressively take advantage of anything that seemed to be a mistake, aka not give him the benefit of the doubt on complicated positions, and if my opponent was 1200 I'd still avoid playing aggressively if the position called for quiet moves.
Also as far as your endurance problems I would never eat anything unhealthy during a tournament, only proteins and complex carbohydrates religiously (mixed bean salad, and turkey sandwiches without sauces mainly). I would do my best to get 8 hours of sleep each night and have a light workout each morning if the hotel had an exercise room. Even though my normal lifestyle isn't nearly this good, I'd change my habits for the tournaments to be more competitive.
Another thing I'd attempt to do is get through my first 20 or so moves in under an hour. Even if I didn't know the opening super well I didn't want to waste my energy on the opening phases.
I'd also try to completely avoid calculating when it wasn't my turn, to save energy. I'd get up and walk around to get my blood flowing and to avoid staring at a board for hours in the early rounds. Also consider that you have to calculate many irrelevant lines if you are trying to predict your opponents moves on their turn. Better to let them eliminate those irrelevant lines by picking somethung, and you can just calculate after that.
Based on following these methods and training about 3 hours each day, I broke through to 2200 in exactly one year, playing just 5 tournaments. I didn't lose a single game to anyone under 2200 uscf during that time, and split 5-5 with players at 2200-2300 (was 4-1 in my last 2 tournaments), drew a 2350, and I also beat an IM, lost to an IM, and blundered a won game against a GM (but atleast it was an interesting game and he talked with me quite a bit afterwards). In my final 3 tournaments I closed out quite strongly in the last few rounds since I had plenty of energy, winning money in my last 3 tournaments (even winning an entire section). Beyond just training myself to be a good player, following these methods gave me a decent competitive edge over my competition and led to strong results.