How to be happy
Realize the past can't be changed.
Like ever. Stop reliving what could have been or obsessing over how the situation may have played out differently if you would have gotten one more word in or reacted better.
It didn't work out for a reason. Take the lesson from it and find comfort in knowing that what's meant for you will come in duetime.
And please don't forget to return it by the due time, or you'll pay fines.