A big mistake starting out was assuming trading would be easy, not appreciating the time (years) it needs to mature in mind and game plan. This includes spending so much time playing with uber-complex math formulas (hello "sliding goertzel"!), hoping, while being confused about what is real in the markets and not. Ie. to truly appreciate "trend is your friend", the inherently chaotic nature of the markets and its implications for complex predictions.
Another confusion that lingered for a long time, is assuming "win chance" in trading to be 50/50, while ignoring risk. Truly, one is a function of the other. So a risk of "0" forces a minimal winratio and vica versa.
My biggest mistake, but probably also a great driver along the beaten path, would be my relentless ego to "figure it all out myself". Though, without other ancient sages pointing the way, I would probably still be completely lost in the dark. It's too easy to get lost in complexity. It's like the basis of trading is so simple it's hard to grasp for most folks. Sages say simple things, not to confuse you or appear smart, but because it's the best advice and they've probably tried many of the blind-alleys already.