As human beings I think we pride ourselves on our relationships. I truly believe that we define ourselves by those we are closest to. It is through their eyes that we see ourselves. It is with them that we are as close to being ourselves as we will ever be. I think we all begin every relationship assuming that it will be a long term one. Whether it's a friendship or something more, none of us begin our relationships thinking "Well this will be fun for a little while." We just think "I really like him," or "we have so much in common, I'd love to hang out with her," or whatever the case might be.
I've recently come across the conclusion that some people aren't meant to be in our lives forever. It's a difficult thing to face, but I think it's true. Losing a friendship or relationship with someone, especially someone you were close to, is never easy, but I think it's necessary sometimes.
We all grow and change and are constantly moving forward and sometimes where you are in your life simply isn't compatible with where someone else is. Maybe you started out in the right place together, but somehow along the way your paths veered in opposite directions. But that's okay. We all have our own paths, and maybe it's okay if sometimes those lead us in different directions. There are no guarantees in life, and relationships are not exceptions. Ultimately I think you have to look at failed relationships as you would a past class and look at the other person as a teacher or professor. At the time you took their class, you really needed them. They were there to guide you and if they were good, help you along your path to passing their class in order to graduate and start your career, and ultimately get you to where you want to be in life. But after that class, there's a good chance you won't ever talk to them again, other than perhaps in passing. Past relationships are the same thing. Those relationships helped you get to where you are today, even if they ended somewhere along the way.
You just cant dwell on lost relationships because there's probably a reason those people are not in your life anymore, even if you don't know exactly what that reason is. Just have faith and know that many people will come and go in your life, but if they're meant to be in your life for the long term, then they will be.