I'm slouching my shoulders :(

So, I've noticed I have really bad posture when it comes to my shoulders. I kind of slouch them. I don't mean to, and my mom tells me every time she sees me "Hold your shoulders up!" I'm 15 and I've been doing it my whole life I believe. I just didn't notice until I was about 11. I'm super tiny, I only weigh 90 pounds, so it's hard for me to keep my shoulders from slouching. I'll straighten up my posture every time I notice, but then I start getting back achs and I have to drop them to make the pain go away. You can tell I'm starting to get a hump, and this is very unatractive, what can I do to make it better?! Helpppp!

Most everything we do involves reaching forward. Google "slouching shoulders exercise" and you will find a number of articles and You Tube videos demonstrating exercises that counter the forward movement and help you change your posture.