Man... Let me try to be objective here. First, I'll tell you I've been through something similar: almost 4ys of relationship and then broke up. It was quite different in my case, cause she didn't have all that consideration with me (which I surely deserved, just like you do).
In all those parts you say you messed up, I'm sorry I have to agree with that. It's typical guy behaviour. But don't worry, it doesn't mean "messed for life", just for the moment, the state things are now. Maybe you've confused her a bit.
I think nothing here is up to you. She has to make up her mind by her own means, as you put it. It's absurdly frustrating for you and it causes sickening anxiety, I can even feel it for you, but maybe that's the way it is... What else can you do? Just relax and wait, as all you could have done was done on its due time.
If she kissed another guy soon after the break-up, this doesn't mean much. What means much, tho, is the fact she only kissed YOU before. You can most probably tell she was curious about it, no? It doesn't necessarily mean she loves or even likes him, if we're correct here and it was just out of curiosity. Please, consider that too. And that she may be thinking "is there more beyond him?". Tell me honestly: is there? Maybe yes, and she deserves it (just like you do), maybe no, and she'll find her love for you again or mess things up herself and make it impossible with you and find somebody else who tries.
Let me tell you the keywords to the whole thing. If the worst comes out, you will hear an awful lot of people tell you stuff like "it will go away", "you'll get over it", "time changes everything". All this will cause you no relief, and anything anyone can tell you won't take you away of what you've experienced yourself: almost (or actual) physical pain. BUT, above all, they are right. Time WILL change it, eventually, and it doesn't take long to work its effects. All I or anybody can do is tell you this, and you don't have to believe it: just take it and go on to explore life. I even feel a little uneven here, cause I remember how it was to listen to this and not to give a flip about it. But that's all there's to say, and it is true.
About being best friends: I myself wouldn't do it, for the exact same reasons you put. It's way too hard. But what you two had once is not necessarily gone. Maybe it's the case of letting her walk for some time, maybe a year or two (or less, or more) and maybe you, with another girl in your heart, may come to be great friends with her again, and just that. You don't necessarily have reasons to hate or despise her, since she's only being honest to you and to her.
Let me tell you about the last part: it is possible she doesn't need to realise how great it was what you two had. If you know that, maybe she does too. That's not the point here. The point is IT IS possible for you both to have it again with other partners, I can tell you that. When you're young and had this great big love, all this sound like BS and your love was the greatest of all, but you have a whole life ahead, and you don't know the secrets for that. One of them I know: IT IS not only possible, but also very achievable. I'm living a much better relationship now than with this one I told you about.
And both of you won't forget what you had so far, you staying together for a few more years from now or breaking up now. It'll be called experience and it will help you a lot later.
I'm quite young too (a little older than you) and I'm not talking as Mr Experienced here, on the top of all hills. This just comes from what I've learned so far.