i lied about my age to get a job

so i work at this big retail place and they do background checks and drug tests and all that nonsense. i was just too scared of not getting hired so i told them on the application that i graduated high school last year when it was actually 2 years ago. its like a dumb lie but i keep thinking about how im gonna get found out and lose my job over something stupid... they probably do regular checks and if they find out im gonna look super bad cause everyone thinks im fresh out of school and can barely handle adult stuff... the guilt is killing me though, what kind of person lies about their age for a retail job? i keep trying to return it but its too late now...