Losing your free will and being forced to engage in... intimate relations... with someone you probably aren't attracted to (and whose entire gender you may not be attracted to) can be severely traumatizing. Most canon characters recover relatively easily once exorcised and neuralyzed because removing the wraith and the memories allows the canon to return and erase the bad slash so that it technically never happened.
PTBSS is what happens when the memories are too strong, the bad slash is too horrible, or the canon has been attacked too often in too short a time by slashwraiths. Canon characters affected by PTBSS tend to pop in and out of character, alternately longing for whoever they were slashed with and becoming disgusted, angry, or despondent with the realization of what they've been forced to do.
Characters with PTBSS should be brought to FicPsych, where the full range of medication, therapy, and sedation (if necessary) can be used to bring the characters to the point where they can be neuralyzed and safely returned to their home canons.
Tolkien or Tolkien-based Elves with PTBSS have a special problem if their bad slash involved rape. Ones who have been raped should be brought first to Medical rather than FicPsych, as they can die very quickly once they have become aware of the reality of their situation.