The past has happened and cannot be changed. Too many people get stuck in the past, worrying about what they did or didn't do, what they should have done or not done. So many regrets. Don't be like them!
The future hasn't happened yet - you can make it whatever you want it to be. So many people live in the future, 'As soon as I get that degree/job/girlfriend/ etc, then I will be happy." No, you won't. Don't be like them!
The only reality is now. This is happening, this is the present. It is a present from God.
Enjoy the moment, it will never come back again.