Listen, no one said attaining the fountain of youth would be easy. A properly motivated party has all of eternity to consider and solve these issues. And while the memories might fade - the muscle memory you develop learning skills and other brain development couldn't possibly be wiped out.
Maybe consider detailed journaling - and once time slips into the modern era, record video recaps. It'll be like in Severance, an outie getting a video message from their innie and vice versa; or like Drew Barrymore catching up with a video recap of her whole life thus far, as in 50 First Dates. Establish the mental pathways in your earth brain with new memories of LEARNING what happened in Narnia, before you fully lose your Narnia memories.