Not appreciating my own accomplishments. I was always the least talented dude in my scene so whenever I did something I never really thought it was a big deal. Looking back, I can't believe I didn't appreciate some of the shit I did, as I will never ever even attempt those tricks again.
Also, never waxing anything ever and always thinking, "Just go faster" makes my life harder than necessary. Overwaxing is a bad habit, but being too "core" to ever use wax makes skating harder than it needs to be. A bit of wax can really help sometimes.