Does anyone else ever have trouble fully grasping the horror of 9/11?

We discussed 9/11 in one of my college cles today and I felt like I could never fully grasp the horror and shock of that day. I was only 11 at the time, so I couldn't understand it then and I feel like I can't fully understand it now. It is hard for me to believe that such a day actually occurred. It's surreal. Can someone who is older tell me what that day was like? Panic? Nausea? Anger? Shock? What did you feel?