Breaking 80. I broke 90 pretty quickly after I took up the game again (I quit after high school and didn't play again until I was 28), because I already had a basic knowledge from my caddying days. It took me a long time to get under 80, and it was all in my head. I would become aware at some point of the round that I was on track and it just derailed me. One day, I was 2 over after 12 holes. I said to myself on the 13th tee that I only needed to make 5 bogies and a par going in and I shoot 79. What kind of thinking is that? But it happened all the time.
I finally did it in a Club Championship A flight final (match play) because I was so focused on every single shot that I was never aware of what my score was. I ended up shooting a 78, but I lost the match on the 19th hole. Bittersweet, to say the least.