Is Masturbation a mortal sin?

Okay, I'm not gonna lie, I've done it more than once. I'm a devout Roman Catholic, and I started reading stuff online about how masturbation is a sin and God will smite me if I do it. I need to know if I need to go to confession and tell the priest so I can be absolved. And please no one say I shouldn't be worried because God doesn't exist, because to me He does!