Is it possible to work again through lessons I have already completed?
Only by deleting the entire tree, which also deletes all your XP and crowns in the language, and you probably don't want to do that. OR by starting a new account and having a second go on that from scratch. You could test out of the easier levels of the skills to get straight to the harder levels if you wanted to.
I'm doing the latter and I'm finding it extremely helpful. It's the only way to get access to the more testing 4th to 5th crown level questions again, because once you have achieved the 5th crown you're thrown back to silly easy lessons where you wast time clicking on a picture of a ball and typing words like cat and muc.