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.
Letting go and moving on completely is a process and often it takes longer for the heart and the emotions to catch up with the head.
The thinking part of us can comprehend and reason that we are making the right choice, it is our hearts which need more time to heal and catch up with what our heads already know.
You are perfectly describing the dilemma which all of us struggle with and eventually face when making the decision to end it with a PD (going complete NC).
If it were a smooth easy transition and a cut and dry decision, we would just do it and make it without any inner turmoil.
This is tough to do. Ending any longterm relationship where we have a long history with the person. You know you are making the right decison, now you just need to be gentle with yourself while the emotional piece begins to kick in and join in with your rational thinking.
NC is the only way to end a relationship with a PD, any contact, generally keeps the fantasy, hope, and illusions alive.
Cut your losses and go through that initial discomfort, which eventually will end, and give yourself a chance at at peaceful and prosperous life.
For additonal support, to join one of our support groups, or to speak with Goldie directly, http://www.lisaescott.com/blog/goldie