Hi, I'd like to help you with your question.
You witnessed a very traumatic event that most likely caused PTSD symptoms for you. Anytime you are witness to violence such as you were, it would be unusual for you not to react with symptoms.
Trouble sleeping and flashbacks of the events are trauma responses. Cutting is a way of trying to control your feelings, which you may feel are overwhelming. It is also a way to feel, even if it is painful. It brings your emotions under your control and you can be the cause of your feelings rather than something like the attack, which is out of your control. You may be feeling this at a deeper level because of the previous trauma in your life. It can reawaken feelings or unresolved issues and make you feel vulnerable again.
The most important thing for you to do at this point is to reach out for help and support. Do you see a therapist? If not, it may be a good time for you to talk to someone. To find a therapist, ask your doctor for a referral. Or search on line at http://www.bacp.co.uk/.
Consider support groups, including in person and on line. The support can make a big difference in helping you feel less alone.
Also, learn what you can about PTSD. The more you know, the better equipped you are to cope. Here are some resources to help you:
The PTSD Workbook: Simple, Effective Techniques for Overcoming Traumatic Stress Symptoms by Mary Beth Williams and Soili Poijula
Mind-Body Workbook for PTSD: A 10-Week Program for Healing After Trauma by Stanley H., M.D. Block and XXXXX XXXXX Block
You can find these books on Amazon.com or your local library may have them for you.
If you start to feel you cannot cope and want to hurt yourself, please seek help immediately. Have emergency contact numbers nearby and a list of family and friends you can contact in case you start to feel overwhelmed.
I hope this helps you,