How to deal with intimidation factor

How can I learn to deal with the majority of players I play in rapid playing much, much faster (rarely ever thinking) than me and yet making fewer mistakes? It’s so intimidating when my opponents play every move nearly instantly, while I’m taking my time and thinking through my moves and how they might reply. It often leads to me making mental errors. The majority of my mistakes seem to be due to me losing concentration and not thinking enough, yet I play far slower than most of my opponents, who typically do not make nearly as many errors. It’s so frustrating and makes me feel like there is almost no point in playing.

Are there any strategies to deal with this intimidating playing style so many people use here? It feels like I have to be way, way better than my opponents to win. I have to fight through moment after moment of remembering to check every little thing, while my opponents can just play blissfully never making significant errors and wait for me to lose concentration. I really don’t want to go the route of having to memorize tons of opening theory, like most people apparently do, so they can play like that. I would rather improve by learning things like tactics and principles and endgames, but if I’m gonna do that, it seems like I first have to learn to deal with the mental side of intimidating ultra-fast play. Any tips?