I had this theory in the back of my head for the like 3 years now. I'm sure many others already figured it out, but seeing how now seems to bring this up, I guess I'll do it.
According to psykos, in order to aquire power beyond your limit you must over come the hellish trail of a near death experience. Her claim is corroborated by ONE and Murata by showing garou as an example. He's faced many hopelessly one-sided battles but always comes out on top due to sheer determination, willpower and skill. The first 2 factors being the most important. Dr. Genus also said something similar about Saitama, an ordinary man with and ordinary life. He's nothing remarkable but through effort alone he broke his limiter.
Garou has gone through a lot of shit to get to his final form, every time he was pushed to the edge, he became much, much stronger. e.g. he had trouble fighting royal ripper at first but later one shotted him after he regained consciousness. During his fight with dark shine it's unimaginable how he could fight, much less move after the injuries he sustained fighting him. Towards the end of the fight however, we see garou quickly overpowering dark shine and even a panel saying "Garou was starting to break his limiter" keyword, starting. He hasn't broken it yet, and this process only started after all the shit he's been through. His limiter slowly breaking gave him so much power his previous self just a second ago can't even hold a candle his new self. And garou's new self wasn't even worth a serious punch from Saitama.
So theoretically Saitama must've went through even worser struggles than garou. And he must've overcame them multiple times, again and again. A lot of people seem to misunderstand saitama's character, they think he's just a guy who did push ups and gained the power of a god just for laughs. I guess that's the first impression he gives off but we quickly see that Saitama isn't just some lucky dumbass who isn't aware of anyone around him.
I'm sure ONE will delve deeper into Saitama's 3 year training period in the upcoming arcs, and we'll hopefully see what he went through to gain his power. Because even during his training arc he was already faster than light and one shotting demon level threats.
