You're grieving the person, the shared future, the version of yourself that existed inside that relationship, and often a whole social world.
It's complicated by the fact that they still exist and can still hurt you, and by everyone's assumption that you should be relieved.
Ambivalence is normal: missing someone you also chose to leave is not a contradiction.
Rebuild routines and identity deliberately. Loss of the daily architecture is often harder than loss of the person.