Well the girl I liked has been going out with one of my best friend.
Have you talked to your friend about the girl that you liked?
No, I was too childish and immature.
It's okay that you feel slighted that your friend is now dating a girl that you like. Friends should support one another and be mindful of actions that can affect other friends. Even though you are hesitant to speak with your friend it would be a good idea to let him about your feelings and give him a chance to apologize.
I think I better leave it and move on.
You can do that too if you're okay. There are plenty of other girls out there.