Lack of experience and proper training. No, a simulator is not a substitute for an instructor. Don't worry though, you're not alone. The Super Cub was my first plane and it lasted about two months before it became completely ruined.
I then went to an AMA club and got stick time on one of their trainers. It wasn't long after that and I was flying a PTS Mustang. A few summer's later and now I can fly quite an array of airplanes, including two I built from balsa kits. There was no quick and easy route.