I turn 47 this year. As I tick up the right-hand side of the happiness curve (which bottoms out at 44--see previous post, Forties Slump,) the major emotion I feel is gratitude. After 45, you take nothing for granted.
Already peers are dying from breast cancer and heart attacks, predeceasing their elderly parents in some cases, so you become aware that it can happen and if it does, no one will stand by your graveside murmuring how young you were and how you died before your time.
So you feel grateful for every good night's sleep, for every meal that doesn't give you indigestion, for every pain-free movement, for every moment of inspiration. Sometimes you forget you're not twenty any more, but you never forget to be grateful.
But most of all you're grateful that you're no longer that young, unaware, ungrateful version of yourself, and that you can never be that way again, now.