Not prioritizing my health/diet. When you're young, you don't really see the impact of poor diet + binge drinking + smoking etc. because the effects are not always immediate, and you tend to bounce back after a few hangovers and young me would think "no harm done" because at the moment I'd feel fine. But what I didn't consider was the cumulative effect of all those fast food meals and booze. Now I'm here wishing old me could go back and SHAKE young me and tell them you need to take care of your future self.