Line on abdomen

As long as I can remember, I have had a line on my stomach that is horizontal in between my boobs and my belly button from slouching too much. How do I get this to go away?? I don’t even wear bikinis anymore because you can see it, and I’ve gained a little bit of weight. When I sit down, even if I sit up straight it has turned into a tiny fat roll. I am young and just want to be like other skinny girls…