I'm really glad you are considering this as a part of your overall treatment. Are you working with a professional therapist or other mental health assistance at this time? They might be able to help you come up with some areas of your treatment that working with horses could really benefit. I am a child of abuse myself, and I find that horses help me feel much more confident and sure of myself, and the effects last for a long time.
One really easy, simple exercise could be just breathing and reminding yourself to stay present in the moment, as a form of meditation - tensing your muscles, then relaxing them. Concentrating on keeping your stomach and hips loose and moving with the movement of the horse might help you achieve better relaxation.
One thing I have seen from trauma victims is a hard time dealing with a loss of control - why a lot of people turn to substance abuse, eating disorders, and the like when attempting to deal with trauma on their own. I personally deal with binge eating disorder - something I picked up from my mother (also my abuser). I find that learning to give up control voluntarily, and knowing that you will be safe, are things that you can easily work with on horseback. If you have a gentle, willing horse that lunges easily, maybe get a friend to have you work on a lunge line without use of reins. I'd try that after you are comfortable with just breathing and moving with the horse, and start at a walk. Once you are comfortable there, work up to doing walk-trot transitions, and so on.