Hey there WX3,
I can totally relate to your situation. The same EXACT same situation happened to me when I was in High School. Let me give you some advice about NC.
The hardest part about that is when the other person just wont let you move on. (aka you block them on IM, etc and they will try and talk to you from other screen names,etc). This is when you have to be your strongest.
I know that this is really hard for you, considering you really still like this girl, but the NC thing is honestly in your best interest to help you get through this situation.
Ok so here is the advice, You need to tell this girl that you blocking her on IM has nothing to do with you not caring about her anymore. It is the only way that you can start to live your own life for now. She has made the desicion to date this other guy for now, and you need to live your own life. Tell her its hard for you to constantly talk to her because you know that you cant have her right now. You still care about her, but this is what is best for you. ASK her to not talk or contact you. That will help you through this whole thing.
Now what will happen, is she will try and continue to contact you, telling you things like you said "I still care about you, why are you doing this, do it if you have to but I still have feelings for you, etc."
YOU then tell her thanks for sharing her feelings, but actions speak louder than words and you can't continue talking to her because it only hurts you. Tell her that she can come talk to you once her and her boyfriend BREAK-UP. ONLY then, will you work on building a NEW relationship with her. You see, building a relationship off of what yall had will never work... look at all the drama that has happened. Have you realized that until she CHOOSES to break up with her current bf, you will always be waiting. HOW LONG DO YOU WANT TO SIT AROUND AND WAIT?! Tell her again to please not contact you until she has broken up with her boyfriend. That is what is fair to you.
WX3, if she really want to be with you, she will ONLY then realize what she is missing and maybe make a decision. If she doesnt, then you have your answer and she didnt really like you as much as you thought. She will never realize how much she is missing until you are gone. Right now, you are still there and she has the both of you. That is why she gets so upset when you tell her you cant talk. Quit giving her what she wants, and do what is best for you. MAKE HER MAKE A DECISION, and in the meantime, if she doesnt make the decision you wanted, well then you know that you didnt need her anyway.
MOST IMPORTANTLY, I know you are young, but please take care of yourself. Dont let this girl manipulate you. DO what is best for you, and in the meantime the best situation will happen to you. Sometimes what we think is best, is far from it.
Be strong and dont go back and forth with the talking, make a stand and continue to tell her to stop talking to you until she breaks up with her current bf. If she keeps trying to contact you, keep telling her to stop. This isnt so you are mean to her, this is so you are taking care of YOURSELF. That is what you deserve. She wont like it, but right now its what you and her need!!!!!
I hope you will listen, and please let me know how is goes. I want to help you through this because i definitely have been there before.