My world changed because of u

Well, let's see...

Anatomy: The head leans forward too much and the neck is too thin, for some reasons she reminds me of E.T.

The left arm is definitely too short, the right forearm also is a little too long.
The cleavage is also strange, because the right breast seems... squished by the left one, which shouldn't be that large.
The waist shouldn't be drawn that way, it seems like she's lacking a rib cage. The navel is not that high, it should be drawn more downwards.
The eyes... It seems like they're melting off her face. Also I don't know if she's looking at the viewer or at the butterfly. Well, at least I like the lashes and the expression, they're nice. :I

Shades: The shades are flat and doesn't blend very well, especially on the breasts zone. The shaded zone should be between them, and yet it shouldn't be that dark.
The lightning on the hair is wrong, it shoud be on the head area and not on the back hair too. The paint strokes on the hair also seems quite random.

Background: The background... Is strange. From what I've understood, the mood of the picture is relaxed and has something to do with love, then why there's that depressing black behind her? Also, the cloud which she's sitting on doesn't have physicity. It seems like she's sitting on nothing.
Long story short, black and blue doesn't blend very well together. At least, not in this picture.

Well, overall, I think you definitely need to learn anatomy. Try to use real life references, or images from Google, there's plenty of anatomy and shading tutorials on DeviantART too.

This wasn't meant to be a rude critique, I tried to give you some useful tips...

I hope this will help you. :3