Consistency in my swing. I've been playing since about 2008 but I would only play about half a dozen times per year. I think I have played more rounds over the past few months than I played in the previous 5 years combined, so I'm finally starting to work on a swing.
I knew my swing plane was funky, lacked power, and was inconsistent. I've never taken lessons so I went to YouTube. Found the below video with a drill that helped me hit my irons at the range yesterday better than I ever have. I've found other helpful vids on their channel, so I figured it was worth linking for others who may not know about them.