That's normal... Preventive measures are - u shouldn't depend or trust someone so much that one even one small change in their behaviour or even breakup that can put tears in ur eyes....
Well now that it's happened-
Give yourself time... Accept that she is not going to come in your life now ... Forget the past... Take time out for you... Talk with ur family or friends or any others who can support you... Try to get regular sleep and exercise... Engage urself in what u love - activities hobbies...
Well what I think is - after a breakup, the loyal ones stay single and heals the damage caused... U have to deal with that..
You will not be able to forget her.
But you can try to accept it that she is not a part of your life anymore.
Try to meet new people maybe?