It is hard to help if I don't know what is going on, but I understand your need to keep it to yourself until you are ready. The best I can tell you is that you can push whatever it is aside if it is non life threatening and if you can eventually share it, at least with your therapist. If it is traumatizing, it is going to affect your ability to deal with your other issues because you will not be able to completely resolve your feelings. This incident will keep bringing up your feelings until you have dealt with it. And cutting may be one of them if that is the only way you have to release how you feel.
You might want to try writing it down or telling someone who you can trust so you can at least get it off your chest and out into the open a little bit. It may help you feel less burdened until you are ready to work on it with your therapist. Also, try your regular coping mechanisms and the ones we have talked about. They will help you find alternatives to cope with how you feel until you can share what is going on.