The Hard Way
First and foremost is to both accept and appreciate the impermanence of feelings, emotions, people, and things. Things change, people change, your feelings change, the way you think, and most of all YOU change.
The biggest frustration and confusion in my life is trying to understand what is really real. How can I go from hating to loving someone else in what seems like minute to minute - myself included. How can something that at the time seems like the worst thing that has ever happened to me be, in reflection, just a glimmer and almost funny? Conversely how can what seemed to be the best thing that has happened to me - in further reflection - seem to be a bad thing?
So when you are in what seems like a great experience - relish it, take it in, and enjoy it. It might not happen again.
So when you are in what seems like a terrible experience - remember all the times you felt the same and remember that it goes away.