Most Helpful Girl
It is very difficult to find closure when you have no catalyst for the termination of the relationship and when you have no one to obtain answers from. The best you can do is just try and accept it and realise she wasn't the person you thought she was, and if she wasn't then it is no surprise that you don't know why she left.
I know it's hard but many people before you have gone through this and many will again, you probably will again - heartbreak is all part of life, we accept the loss because we know there is always the possibility of another love, a greater love. It seems dark right now but it will get better. Try and distract yourself or do some serious introspection to figure out why things didn't work and how you can come out a stronger person for it.