Reading this reminded me of me and my boyfriend in a weird way. We'd been together about 2 and a half years, both of us being seventeen. We were extremely close, he was my best friend as well as lover, i lost my virginity to him (as did he), and we'd been through a ton.
I depended on him. For everything. And I told him that all the time, and we had talked about how hard it would be to break up.
And believe me, it was hard on both of us. He made the ultimate decision, and I know I didn't make it easy (still aren't

I gotta keep him on his toes haha). But I realize that he needed out, and really I needed out too. When you're a teenager, you need to be yourself first and foremost, and being me+him for 2 and a half years wasn't doing that for me.
She will cry, she will think she cannot live without you, but if she doesn't have any serious problems, she will realize that she will persevere. And she will be OK.
And remain friends with her afterwards. No matter how many times she says she can't be your friend, shove your way in. She needs you now, if not more than ever. If you love her, you will be there for her.
So, just talk to her about it. Don't change your mind about your decision, go in knowing that you're going to end it, none of that yo-yo stuff. And do it in person for heaven's sake, if she cries, cry with her. If she needs held, hold her. You need to accept the consequences of your decision, but don't fall back on it out of pity. Let her talk bad about you, let her fight you, let her do whatever it is she needs, and you just remain her friend.
You will be thankful in the end.