Like it Feels to be Substituted

It is about being replaced by someone else.

It's like C, a girl, has been friends with A, the boy, for a very long time. Everything is good and going smoothly. There is a possibility that A and C would fall in love with another.
All of a sudden, another girl, B, comes in between A and C .
A didn't tell C about the girl.
C made a mistake of thinking it wouldn't change how things were between them but it did.
So, slowly A and C became strangers.
One day C calls, saying she wanted to see him to spend time and to catch up with him but A says he has a commitment with B ( that's the other girl in the equation).
It came as a shocker because C never expected it.
That's it .