The book "Mind Gym" helped my brother who's a professional gamer (online strategy games). It examines some of the ways that athletes can activate and harness their psychology to do better.

One thing in particular that helped me in competitive fields is to visualize in advance yourself winning: but not just winning, how you win. Visualize in color and detail how you're ready for everything, how you will deal with each challenge how you will persevere and keep focused throughout. 

If you've already won once in your head, you won't be surprised that things go well and you won't sabotage yourself.

Everybody has their hangups and this is purely psychological. But the psychological part is very powerful. It is actually a common problem for high performers that they'll do much worse against lower rated players/teams.

Practice more than anything can acclimate you to this. Practice against worse players. Practice against the players you want to be able to beat. Lesser quality players play markedly differently and it's possible that you've only mastered the high level kind of play and although you should have all the chess tools in house to beat them, you may be missing the right mentality and this might need some figuring out and some time.