I've come to realize that the success of a relationship is not necessarily contingent upon how long it lasts, and I've mostly been monogamous my whole life, and practicing poly since late 2010. While it can be a very nice and comforting thing to grow and deepen with someone over time, sometimes relationships that are quite short in duration can teach us a great deal and enhance our lives in unexpected, wonderful ways. At least, that has been the case in my life. Some of my most memorable and life-altering relationships were only a few months long. However, that doesn't mean that I have not been in anguish many times in my life because a relationship ended. Of course, we want to see a relationship continue if we have feelings of love or fondness for someone and it brings us joy.
I was devastated completely when my marriage of almost 12 years ended, but I think a huge contributing factor in my devastation was the expectation that we'd be together forever. I was wrapped up in the sense that I was a failure and had forgotten what I'd always said when I was single, and often to my single friends when I was married, that just because a relationship ends doesn't mean it failed. All it means is that it ended. Some relationships are not really meant to last or shouldn't, and can still be enriching if they break up.
And this refers to friendships as well as love relationships. If a friendship with someone discontinues, everyone will eventually get over it and move on. Such is life. All you can do is stay as aware as possible of your own internal processes, and treat people the way you want to be treated. There are no guarantees in life.