I'm feeling a bit like that with my writing at the moment. I'm just coming to the end of a 12-week on-line Open University poetry course called Start Writing Poetry. Boy was it hard! I've never written poetry before, other than the occasional comic verse and a string of haikus, so it was all new to me. I'd never even heard of a vilanelle or a sestina, and here I was writing them, along with freer non-rhyming poetry and sonnets.
But the interesting thing is, even though I haden't been working on my novel to speak of over December, I just sat down last weeked and got straight back into it and it seemed, if not exactly easy, then easier than before.
I think for me poetry was that next level up and having struggled to do something much harder, it's developed my writing muscles and made the fiction seem easier by comparison.