A steel beam from the World Trade Center’s fallen South Tower was displayed Monday at a Long Island education center, where hundreds of families devastated by 9/11 tearfully flocked to the mobile memorial. “We can’t forget,” said a weeping Rosemary Cain, the mother of slain Sept. 11 FDNY firefighter George Cain, to The Post. Cain was brought to tears at the sight of the 21-foot-long, 16,000-pound steel beam, which a police pipe band somberly escorted as it was hauled to the Lieutenant Michael P. Murphy Navy SEAL Museum in Suffolk County on a special truck as part of a cross-country tour.  Several Long Islanders paid their respects to a beam from the World Trade Center on Monday. John Roca Cain recalled how her 35-year-old son was a “happy bachelor” and avid skier who would have turned 60 next week, had it not been for the 2001 terror attack. The Ladder 7 Bravest was killed by the collapsing skyscraper while evacuating the nearby Marriott Downtown hotel with members of his 29th Street firehouse.…