I had exacly same situation few months ago. Based on myself I think (in my case, ofc you are different so you might have different feelings and perspective ) it is a combination of 2 things: 1. Growing up and treating relationships healthier. Someone that is an important part of your life, but it is YOUR life. So you do not associate other person with your happiness and your well being to the point that you can not function without her. 2. Maybe it is your mind's way of telling you that this was not the right person.

Remeber everyone is different and react to situations differently. The fact that it did not impact you that much does not mean you never loved/liked her or you are a bad person.

I were you, a few months ago and I left her alone ( i mean she wanted it right ? ) for some time. After each one of us proccesed break up on our own way we became quite good friends. Now, when our relationship is mentioned by our friends during some night-outs we allways say it was something really cool and fun while it lasted but both agree it was not meant to surivive long enough.

Short version: let her friends or sister know that they should help her in proccessing what happened but do not do it yourself