I'm so sorry this has happened. It really hurts when someone you had feelings for suddenly seems not to care any more.
To answer your question, the reason you are finding it so hard is that your feelings were developing and getting stronger and all this came very out of the blue. The reason that (much to your bafflement) she seems to be able to switch it all off like a tap, is because she has probably been having a few doubts for a little while, so she has been able to distance herself, make the decision to end it, and then tell you. She is already part way through the grieving process (much closer to the acceptance stage) whereas you are right at the start of it.
Although people will say 'just move on', it really is like a grieving process. You are grieving for the strong feelings you had, for what might have been, and for the sudden change in your life and free time. The best advice will be just to give yourself time. You will be upset, that's natural. But even though this relationship hasn't gone the way you wanted, you have hopefully also learned some good things from it such as how to love and be close to someone and share your life with someone. Those things will be good for you to know down the line, even though they have come to such an abrupt end this time.
Sorry again.