Someone once told me...
"What it means to be truly happy... is to be grateful. To appreciate what you have now. Don't think of what it could've been, or what it could be. See the present, and live in the moment. What happened before is in the past, and the future is unknown. No amount of regretting will change anything. The past doesn't set the future in stone either. No matter how hard you try to predict the future, you just can't be certain. Anything can happen. Live without certainty, live in the moment. That's the fundamental key to happiness..."
-A very wise commenter.