First I want to say...as a nursing student myself, who also took organic chem during summer session...GOOD LUCK TO YOU! Seriously...chem (or upper division math) during summer is HARD!!! With or without dissociation issues.

You have a few options avalable. My first suggestion is to go to your school's disabled students resources department and do an intake interview...get yourself set-up with them. Disabled student resources can provide you with accomodations specific to your needs, that will help you with your classes. I have been going through them for the last 2 years and it has helped a great deal. Things that would be helpful are a quiet room for exams, extra time (usually time and a half for dissociative type things), notetaking, tape recorded lectures, etc. The way note taking works is disabled students provides the teacher with self-carbon note paper...the teacher announces to the class that he needs copies of someones notes for a student that has difficulty taking notes (but does not say who). A student takes notes with that special paper, then gives the copies to the teacher...the teacher then gives the copies to you at the end of class (in office or after everyone has left the room, or whatever you agree on). I actually have never used the notetaker accomodation, but I have used the others I listed. It really does help.

Is this your first class? If it's not, is there something in the room, about the teacher, the topic, another student, that may be triggering you?