I am a self taught golfer as well, but throughout the short period I have been playing (around 10 months), I would have a great round (10 over) then the very next day I could shoot 18+ over. My good shots were great, but I was really punished for my poor ones. I thus decided to get a lesson.
The first lesson was just about my set-up. My instructor (Andrew at Melbourne Golf Academy, in Melbourne, Victoria, Aus- top bloke) watched me swing and hit a few balls, filmed my swing a couple of times, then showed me how my stance was forcing my backswing to a really flat trajectory.
He then showed me a couple of easy to remember tips for the correct stance, and the correct swingplane my club should be taking. After that one lesson, I went out and shot the best score of my life at a track I've played a good dozen times. I've noticed MUCH more consistency in my swing; scores getting lower; and much lower blood pressure having pretty much eliminated the power-slice from my game. And that's after one lesson! I need to go back and have another just to see how much more improvement I can see!! I can't recommend having a lesson highly enough it really helped my game.