[....]
However, what makes Man United superior is their mental condition because they don't have a bigger burden than Everton. Everton's demands are currently out of the relegation zone so at least that will affect concentration on the pitch.
I can agree that MU players are better equipped mentally but not because of your reasoning that they have less burden to carry. Do not forget that they are one of the biggest club in the world and majority of their huge fan base probably expect them to finish in the top 4 at least. That alone is a bigger pressure to win most games specially against smaller clubs like Everton.