Welcome to the forums. I'm sorry for the pain you find yourself in, nothing ever hurts quite so much as losing your first love. I can understand why you are asking for help here as I am sure that how you're feeling about needing your ex-girlfriend tracks pretty closely a lot of feelings that members here discuss about their therapists. In many of our cases, those feelings about our therapists end up being feelings about unfulfilled needs and longings from childhood. I don't know if that is the case with your girlfriend or not. If you are feeling really paralyzed by this and unable to get past it, you may wish to consult a therapist to see if talking it through might help you understand why there is such longing and dread involved.
Beyond that it's difficult to go, as the only relationships we talk about in general, are therapeutic ones and our other relationships as they relate to our work in therapy. Not having met you or your ex-girlfriend, I would very much hesitate to give you specfic advice as to how to handle this.
But it sounds like you are in a lot pain, and i hope that you can find some relief.
AG