The game will always feel easy to you because you're the one making it. You know exactly how it works and what you're supposed to do, and you're constantly practicing it every time you test it. I think that's why NES games were so notoriously difficult, they had small teams with not many outside playtesters. I think as a rule of thumb it's better to err on the side of making it too easy for you, if you want it to be reasonable for someone else's first play through