I always fantasize about being really pretty like the girls on instagram
That's not too unusual. Do you find your self thinking about this often?
I'm really desperate for this because I'm not pretty enough.
You shouldn't be judging yourself against instragram models. They go out of their way to take flattering angled photos so you don't get the entire truth of how they look.
I don't care. I need to get plastic surgery done.
Don't jump to conclusions about needing plastic surgery based on what those girls look like in their pictures. They use all kinds of software to trim their body parts after taking the picture. You would probably think they looked very similar to you if you saw them in real life.