friends are suppose to be there for you

Today was not a good day for me. I recently found out and i know this is going to sound like high school drama which it is but i dont really pick good friends. This friend who i've known since my senior year in high school who is friends with my sister, decided that she didnt want my sister and i at her 18th birthday and i 've been told i didnt really socialize with people at her last party , well i didnt know anyone and i dont go to high school anymore so dont expect me to me all chatty kathy. How do i talk to a group of people i dont talk to, dont hang out with, or go to school with? Basically this friend is annoyed with the fact that i'm unhappy. Well isnt that what a friend is for, to help you get through the hard times, suppose to be there for you, my happiness should matter to my friends, theirs matters to me. I remember i would sit constantly and listen to my friend whine about someone she liked or getting dumped or chatting about how she doesnt like people. I dont think its fair to me that she can be upset most of the time but i cant. That is not how friendships work. I was so angry, i never thought i be betrayed like this but then i thought, why should i even care? she hasnt been there for me when i needed her and its just a birthday party, come on i'm not in high school, i'm growing up, it sounds silly but i cant still get over it you know? i'm going to try my hardest because i will not let her get me down. Soon i will be going back to school and making new friends. I have this support system who i know there are people that would want to listen.  I never realized until now that i chose the wrong friends, i guess it took the pain and betrayal to realize it. I'm done with this person, i cant even talk to her. I lost a friend, its just one person. I'm happy just hanging out with my sister. So i'm moving on.