(This has been said a lot, here nd otherwise. Sorry, but it needs restatement.) Magic events are long and grueling, and they take a physical toll on you as well. You might think that focus is only mental, but your physical state affects it as well. Doing something as simple as staying hydrated helps. Water is cool like that, and it lets you focus on the game rather than how hungry/thirsty/tired you are. Drink and eat well.

The other part of it is mindset, and mental toughness. I used to play competative chess in high school, and I feel it really helped me focus in magic tournaments. In chess, you have to make zero mistakes for upwards of two hours to win. For one game. I don't mind playing magic for hours on end partially because I'm used to having to focus so much. I'd suggest giving it a try (play against someone using clocks).