This is happened to DD and myself, too. Growing up it would happen when friends met boys and merged into couples. Then I did it and still get dinged by a friend who did the same thing.
There is a friend at DDs school who bounces between other kids and DD will get dumped, but what I learned is that it's the parent. This is the type of person who seeks people for their resources, not so much for real friendship and that helped to learn.
I don't let us get too attached really to anyone now and it really helps, but still hurt can come.
I'm really sorry, JRC. We get to the end of our rope at times, don't we?
You gave me nothing and now it's all I've got - Bono
A person is a person, no matter how small. -Dr. Suess
Perserverance and spirit have done wonders in all ages.