I'm annoyed by my girlfriend's zest for life
Why would you find that annoying?
She's always doing crazy things at the spur of the moment.
I think maybe you should learn from her approach and lighten up a bit. You can't be going around telling other people, especially your partner, how to act. She'll think you're being controlling and probably leave you.
Maybe I'll try to lighten up.
Good I hope you do! I'm sure she would like that a lot as well.