Adding onto the above, instead of going back to old lessons, just learn new songs using the "Songs Made Easy" tab, or practice chord changes with a metronome. You shouldn't really need to go back to old lessons, because songs that you learn will incorporate what you need and naturally retain them while learning new things at the same time. You don't need to fully master anything, retain everything perfectly in guitar fundamentals, nor do you need to stress yourself with constantly going back to old lessons. Just move on and most importantly have fun :)