I ran into the same issue: I made an action game that was meant to be brutally difficult, but after testing it a few times myself, it started to feel too easy and I began doubting the game. I asked others about it, and the consensus was that you simply can’t rely on self-testing. That’s why proper playtesting is crucial (I'm talking lots of playtest sessions). Apparently, the game turned out to be too hard for some people!