It seems as she had the chance to realise what was failing in your relationship along this time you’ve been far.

Sometimes, a part of the couple screwed things up and the other party sees her/his feelings changed in the way that it cannot be undone anymore but you can still learn a lesson for a future relationship with somebody else.
Nonetheless, I would give myself the opportunity to have at least, a last conversation with her (always if she wants it too) so you will both may see what you feel, guys, to each other. And at the very least, tell each other what you both need to tell.

As to this realisation you had, in the sense that you needed to change what you did wrong, exactly, when happens the point in which you realised of it? Now, that she’s gone? Did she called your attention at this issue before? If so, what was your reaction and your actions? Did you already make any different from the time you proposed you to change things?
These are questions for you.
I don’t have the intention to subject you to third degree. I’m only a little sceptical about us being able to make big changes from one day to another.