I don't think you're acting like a jr high school girl. I would of felt the exact same way and I'll be 41 yrs old next week! I think that no matter how old we get we still can have our feelings hurt. I really feel your pain in this. I'm not good at all at making friends and I have only one lady that I talk to quiet often and she and I get together every 2 wks and have lunch or I go to her house and we sit around and talk for a while. She has done stuff to me in the past that really, really hurt. Of course I forgave her but I've never forgot and I don't feel as close to her as I used to feel.
I think that since y'all were making specific plans together that she does owe you an explanation. I would ask her how her trip was and see if she'll bring it up and explain why she didn't take you as planned. If she's really a good friend to you then she should put herself in your shoes and understand that if this were done to her then it would hurt her feelings too. She should have been respectful of your feelings and should have let you know that she changed her mind. Even if those other ladies didn't know you very well and didn't want you to go your friend should have been on your side. She could of declined to go with them and you and her could have went together as planned. I'm interested to hear how she explains herself. I give you big hugs for having to deal with this. I'm so sorry that this happened to you by someone you really thought that you could trust.