A PT once told me - if it pains, don't do it. I know its hard to follow that though, because we end up doing it.

What exercises have you been given to correct the FHP? I suspect cervical retraction. It will cause pain in the back of the neck, because what you may end up doing is translation, i.e, you are moving the vertebra backwards which stresses the back of the neck and is painful. Instead, do gentle chin nods as versus chin tucks/ backward horizontal movements. Another thing to do would be to massage the back of your neck, just below the hairline. Those muscles are often very tight in those with FHP and perpetuate the issue. Best of luck!