Happy early birthday. I think what you're feeling is pretty normal. I'm turning 40 this year and while it's a nice round birthday, I'm trying not to expect much. My family and are planning to be in Alaska on a family vacation. I can relate to what you said - it would be nice to have something to mark the occasion but would it be enough? What would even be enough?
I think you're spot on that life is an investment. As I'm a man of faith, I believe it goes beyond this life though. I don't know where you stand on that, but I believe what we do in pursuing God and spreading the gospel of Jesus is what will last. The other things of this life will all ultimately pass away.