The biggest challenge I had was taking charge of a project and taking it to production for the first time. It wasn't the technical side so much as bearing the responsibility, and I found myself being overly critical of my own work and getting bogged down in the details, to the point that I had burned myself out and ground to a halt. Fortunately I worked with someone who recognised what was going on and was able to set me straight (I still work with said person). It took a while to get it into my head that a project is never really "finished" and sometimes you just have to hit the button and push it to production.