Help with goodbye ex-best friend "speech" ?

Well, here's my problem. I have been best friend for 7 years with a person who i totally felt it could understand me, feel me and help me whenever i need her. As the motto says time changes best friends become strangers, and that's what kinda happened. A year ago she dumbed me for another circle of friends who were open to drinking beers all day long, smoke stuff and go out till 4 am and more.. Anyway after 1 year of constant fights between me and her, we found our way this summer. On september i had my birthday, and she asked me to go with her to a concert of a friend of her's boyfriend cause that girl didnt want to be alone with him and his band. My relations with this girl are awful. She set up a fight between me and my best friend and she was the main reason we got apart for 1 year, but this don't cover my best friends will and actions. I answered of course no because i wouldn't even go out with that girl out even of a simple walk, and that it was ridiculous to ask me to p my birthday that person. In the end my best friend told me that she knew i would say. she added that be she had already promised to her friend to go to the concert with her and that she couldnt cancel it now. (she didn't even like that girl in reality cause she was a huge lier and her boyfriend's stealer but she accepted to there with her instead of going out with me...) My best friend asked me to out with her at least before the concert and so we did, then she promised she will call me to meet late after the concert to celebrate it. she didn't call. the next day she told me she was sorry but she returned late from that concert. i was sad cause i though that she was joking me and that she wouldnt sell me for a concert of a neighbourhood boy band.. i told her i was dissapointed and that i couldnt expect something better than it. since that day we havent talked. i dont think that i should be the one who needs to apologise or feel sorry, even though some times i tend to blame myself when its not my fault. I am sick and tired of these foolish treatments, she has forgotten me completly and doesnt count me in in anything cause of her new friends and also because she doent think about me at all. im so dissapointed... i always make the first move and tell her what i feel about her and how much she means to me and she agrees with me too but the next moment she completly forgets it.. so what i want help with is a note or a letter in which i can tell her a goodbye that will last because honestly im so sick of it even though im sad about it. i wanna show her that i am right to her but also that i am tired of forgiving everything. could u tell me ur opinion or a possible letter appropriate for my situation? thank you a lot.