This is the nature of grief, loss, and change
We grieve endings whether they were good or bad, it is still going to be a big change for you, as well as a loss. The loss of what "should" have been, never was, the good and the bad. We grieve the death of the hope, fantasy, and illusion of what we wanted it to be.
We fear the unknown and often get comfortable with that which is familar and venturing into the newness and unknown may be challenging on many levels.
Just because a relationship had many hurtful components does not mean that there were not things about being with that person which brought you comfort. As awful as PD's are, in truth it was not all bad or we would not have been with them in the first place.