Terrorism isn't about the frequency of occurrence of terrorist acts, or of similar kinds of attacks made during open war. Londoners of different generations experienced the Blitz and the IRA bombings of the 1980s. Many of them have been through this before. However, it is the very unpredictability of terrorism that makes it so frightening, that makes a return to normalcy as difficult as it was the last time, because the ordinary citizen has no way of knowing when, where or if another attack will happen.
People deal with this in a myriad of ways. Some become defiant, others resigned. Some find themselves swallowing down fear for weeks, months or years after the events, every time they board a bus or enter an Underground station. This is the real point of terrorist attacks, not the body count. All emotional responses are fully permissible, but it is the way that we act upon them that will determine whether or not we build a world in which the slight probability of terrorist attack on the average citizen will continue to be a weapon that can wield so much power.