Q How can it be my fault if I am hit by a car while I am lawfully crossing the street in the crosswalk?
First, the driver may be charged with a traffic offense for failing to yield the right of way to you, the pedestrian in a crosswalk. This means that the driver has "broken the law" and has received a citation to appear in court to answer the charge or pay the fine.
That is different from your being able to hold the driver responsible for your injuries in your own personal complaint against the driver for your injuries. In Virginia, the pedestrian can only recover for her injuries from the driver if (1) he was negligent and (2) she was not negligent in a way that contributed to her injures. That is called contributory negligence, and in Virginia, contributory negligent bars an injured person from being repaid by the driver.