I agree about finding your areas of interest and exploring things that interest you. There's still a couple of decades until retirement. I think the idea of the kids leaving the nest is a bittersweet one because it's the closing of the chapter of a book.
Try to sort through this concept of purpose. I hear that tossed about all to often, not just you, and I get it, it's almost like a metaphor though as though if one doesn't go big and strong there's no point and that's just not true. It's the smaller moments of affecting lives of others that add up in the end. Point of reference is that I hear the purpose driven life rhetoric more from my born again buddies than any other fragment of my irl circle.