It sounds like you are feeling really stale with the lessons you've been working on, so I would go on to the next lesson. It's not as if when you move on to the next lesson you will be abandoning what you were working on. Instead, you'll be incorporating everything you've done so far with something new and fresh. Think of it this way: you'll have a whole new set of problems to deal with!

Seriously though, if you move to the next lesson and immediately feel like you're in over your head, all of those previous lessons are still there. My own pattern is a bit of a revolving door - I move forward and take a lesson or two, but almost always circle back one or two lessons again so I usually am circling between two or three lessons. As I said, my own progress is very slow.