It's always difficult to give up a dream.
A relationship is even more difficult to let go.
You believe you are with someone you will be spending the rest of your life with. For whatever reason they decide their life would be better or happier without you.
Whether a person gets fired from a job or dumped in a relationship, rejection hurts!
However in order for them to have been "the one" they would have had to see you as being "the one" in return. Basically if you're not together then they were not "the one". (At least not in the forever sense)
Everyone changes their idea of what their perfect mate is as they mature and evolve. What you want in a mate at ages 16, 19, 25, ...etc may not be what you value or desire at 35, 45, and so on.
It's important to keep things in perspective after a break up.
There are almost 7 Billion people on the planet.
The odds are in your favor there are thousands if not millions of people you could have a wonderful life with. If there was only "one" person for us the majority of people would be married to their high school sweethearts! Life does go on.
However you still have to go through the pain, do some introspective thinking to figure out how you became more "emotionally invested" then your ex, were their signs you ignored....etc I wrote the following hub awhile back which may give you some additional insight.