As a therapist I have used EMDR Therapy for about 25 years and trained with the accredited EMDR Association and with LCCH. I use it mainly for PTSD and for unblocking. I consistently have dramatic results and never have found you need 12 sessions that would suggest something is not right. It is a very deep and quick therapy typically three or four sessions is enough. I have used it with survivors of child abuse, victims of assaults, car crash victims, soldiers, airline pilots who have had flight emergencies and other traumatic events.
My advice would be to make sure you have a therapist who is trained by an EMDR accredited training body as many other types of therapists do short non EMDR accredited training in EMDR which may not be as thorough. I am just retiring so not taking on anymore trauma patients so check out the BACP and UKCP or with the UK EMDR Association. If a therapist books you in for 12 sessions without even meeting you I would be cautious as that many sessions may not be necessary. Hope this info is useful kind regards Christopher