Really sad..help me please?

I dont have any grandpas and didnt meet any of my grandparents on my dads side (they died) and so did my moms dad but I met him. Today I was at Hallmark looking for a birthday card for my dad and then a cute card caught my eye. I took it and opened it and it was for a grandpa. It hit me then and there: i dont have a grandpa and I never will. Suddenly i sat down and started bawling. I dont normally torture myself like that but I read every single 'to grandpa' card. To top it off, a super sad song was playing. I told my mom afterwards and she also was crying. I lost my last grandpa at the age of five. How do I cope with his death in a mature way?