How can I stop wanting to be her friend?

I was best friends with this girl all last summer up until this past February. We hung out literally every single day and we were like sisters. I'd never had a friend like her, her friendship and our sisterhood meant a lot to me. But in February, a huge fight ended our friendship. We never spoke again except to make little hateful comments at each other here and there. I know there is no way we could ever be friends again. We actually made up about a month after the huge fight, but she had changed a lot. She'd started using drugs and alcohol and partying a lot just in the time we weren't friends anymore, and honestly she had just become a different person. We were only friends again for like two weeks before going back to "hating" each other, so I don't really count it since it was very brief and we hardly talked anyways. That was in late March. And now it's almost July and I still feel miserable. I've healed a little bit I guess, but hardly at all. I honestly miss her so much. I feel like I can't talk to my friends or family about it since I've got them all fooled thinking I hate her. I do hate who she's become and the people she's friends with now. But I find myself so stuck in last summer, always thinking of our memories and the fun times we had. I'm always wishing I could go back. I don't have her on Facebook anymore obviously, but I find myself checking up on her profile pretty much every other day anyways. It's not like I'm trying to stalk her or anything. I just really miss her, but I know there's no way we can ever be friends. I know she hates me. I feel so hopeless and I know I just need a way to get over missing her so much and wishing I could take everything back. I've been so upset and depressed for almost five months now and I just can't take it anymore. I don't know what to do... I miss my sister. But I really wish I could let her go. I keep myself occupied with other friends and activities but nothing seems to work.. how do I let her go?