Devonshire-Delight's advice was brilliant.
I'm also a tall girl, I'm a little over 6ft, and I used to slouch too. I think I did it subconsciously, thinking it would help me blend in, when I suspect the reality is, it made me stand out even more.
I was suffering from backache though, which I knew was related to my slouching, and I did go to the GP about it. I was actually given a sheet of exercises to do, which I had to do gradually, so I do think Devonshire's suggestion of going to see your GP first is spot on.
Over the past year, I've learnt to embrace my height, and I now walk tall with pride. And no backaches.