What am I missing?

I have no idea about what to do with my life.  I am 39 years old and I feel like life has passed me by.  I am thankful for what I have.  I feel like there is something that is missing.  I don't know what it is, but I am wondering what is missing in my life.  I am in need of excitement.  Sometimes, I feel like I have nothing to show for it.  I hope that I am wrong, but that is how I feel at the moment.  It is quite a lonely experience.