Is it immature to ignore a girl after she has rejected you? Read the description?

Anonymous
I really liked this girl at my school. We became friends and eventually I developed feelings for her. I told her I liked her and I wanted to ask her out. She rejected me with "I am too busy with school and I am not looking for anything serious right now". Like a noob, I believed her because she rejected every guy that asked her out with this excuse. Boy was I wrong when she started dating a guy three weeks after she rejected me.
It was not the rejection that hurt, but the fact that she lied to me when I opened up to her. I know why she did it. She wanted to reject me gently but what she failed to understand was the fact that it hurst so much more when you realize that her excuse was a lie.
She wants to stay friends by inviting me to group outings and such. I polite decline every time. I actually hate her a little bit. I mean, could not you not tell me that you did not like in the beginning? Why lie?
Now I ignore her most of the time. I greet her when at school but do not engage into convos.
I think I am acting childish but it is easier for me to ignore her than be friendly and in the friendzone. What do you think, people?
P. S. I recently learned that the guy she is dating actually has a girlfriend so I do not know how that is working out for them. I would not want to be anyone's secondary. Would you?
Updates
+1 y
Is it naive to think that the girl might come around in the future?
Is it immature to ignore a girl after she has rejected you? Read the description?
21 Opinion