The cycle.

Though we do not want things to end this way but honestly speaking, this is the cycle that is inevitable, that all of us would go through/have gone through. How sad this is but the ones who stayed are the ones who you should really treasure.

And of course, if you think that the other matters a lot to you, both should put in the effort to keep it going and not let things end up this way.

You can control how things will end up to be. There's no excuses for that. So, TAKE CONTROL and don't blame it on anyone or anything for the sad ending!
"What's worth having is worth working hard for."