Damn right it's not your responsibility. But more important than that is that you CAN'T know the answers to these questions, it's just not possible. As far as why did she break things off, did she ever care for me, what went wrong, etc.

But maybe instead of filling in the blanks with the worst things you can imagine (she never loved me, she never cared for me, it was all lies, she'll never think about me again, she didn't care if she hurt me, etc), it might bring you more peace to be more rational about it and acknowledge that you don't know any of that. It's just your own anxious style assuming the worst.

There is also a complete disregard for her needs, if what she needed was space and to isolate or to completely move on altogether. You're too busy focusing on YOUR needs and what you needed from her, what you wanted from her, what you expected from her. Like how dare she isolate or move on, after you gave her the privilege of sharing some close times with you. Are you able to see this other side of the coin?

You'll probably never know why she made the choices she has, regarding you and the friendship, but due to the unknowns it's a neutral situation. You can choose to assume the absolute worst about her, or you can choose to assume the absolute best, or you can choose to acknowledge that you just don't know but at least you were able to get some good, shared things out of it while it lasted, things that may have helped you develop or cope in those moments, things that she was able to give to you, and chose to give to you, at those times.

You can have a little collection of good memories to keep as you go forward to experience more good things with new people, or you can choose to tarnish and discount and devalue the whole thing in your mind and go forward with pessimism. Although you should know that if you choose the latter, you are working your way towards becoming someone more like me and your former friend. That's how it happens.