It's a tough situation for sure. Painful and sad. This IS one of the things about poly - more people - more people affected by pain.
But I think it's important to keep some connection to reality and human nature. People often get into relationship to get 'needs' met. But 'needs' are not static. And people are not static. We grow, change, our outlook on life and what's important to us changes etc. In an ideal world this would not affect anyone else. But realistically, it does. And sometimes these changes push us off in whole new directions.
It's nothing about 'poly' really so much. It's just about growing, evolving and living day to day.
Somehow back in the back of our minds we have to keep a connection to this.
And if we are trying to embrace love, then what's often called compersion has to kick in.
Despite how it may feel to us, we have to want people we care about to move forward with the direction of their life that's most satisfying and fulfilling. And sometimes we won't be following that same path.
It is what it is...............