I don't like how we say that any relationship that ends is one that failed.
Because, seriously, if you helped each other be less alone and lonely for a while, or if you helped each other love more deeply or meaningfully, or if you helped each other become stronger, kinder, more thoughtful people — I mean, it sucks when a relationship ends because you couldn't make a long distance thing work, or you just didn't have a lot in common anymore, or someone was stunningly mediocre in bed1, but it's hard to characterize a relationship like that as, all things considered, a failure.
The relationships that fail are the ones that leave its members more thoughtless, more jaded, more isolated, more cynical, and crueler to themselves and others. Or, in other words, the relationships that fail are the ones that leave its members worse than if the relationship never happened.
We often don't have any control over whether a relationship ends, but we do have a lot of control over whether it fails. We should all try harder.
1TOTES HYPOTHETICAL EXAMPLE, LADIES, I SWEAR.