How Do I Get A Flatter Belly? Plllz?

plllz im pretty thin execpt i have to suck my belly in alll the time it dosent hurt me cause ive been doing it for a long time everyday in fact i have such a b ha bbbit that at times i even suck it in when i sleep, i want a naturally flat belly without having to suck it in every second to feel beautiful…i know to do crunches and eat lots of protein but how else plllz illl lov ur help. dont suggest diet p ills im 13 and preet ty thin exept my belly(whern not sucked in) ruins m y pretty girl look. so helop plzzz