Everything has a beginning and an end, but that doesn't make the middle any less significant.
We've all been part of a break-up. Whether it was mutual or not, whether you were dumped or the dumper, there is an urge to judge the entire relationship on that ending. After all, if the relationship was legitimate, why would it not last? Was all the time spent together flawed, or even a hoax? There are moments of doubting whether the love ever existed at all. This is especially true when time hasn't enabled proper reflection on the relationship.
But, perhaps this is the wrong approach. Why should a close relationship ending deteriorate your evaluation of the relationship up to that point? Doing so is akin to judging your entire life based on the last two weeks of your life, or how you died. That isn't reasonable, nor is evaluating an entire relationship on how it ended (or because it lasted or ended). Something in the present can not go back in time and change what once was.
Everyone who has been in a break up can look back on some fond memories of the relationship. Perhaps, the person in your life taught you something about yourself, or helped you through a difficult time (or times), or just made you laugh and feel special. These are the moments that make life wonderful, and it is a shame to diminish these.
Why should any of these moments/memories be less special because the relationship ended? Sometimes, things come to an end, but that doesn't mean they weren't wonderful, and genuine, while they lasted.
Everything has an end, but that ending doesn't have to dampen the entire experience. The experience itself remains, objectively, unchanged.
So why not cherish the relationship for what it was? In a relationship, wonderful moments remain, even when things end.