First off, it is quite a lovely piece. I really like how there is a movement and purpose to it. She's not just statically standing around but you can tell that she is almost creeping up on something, trying to stay quite. The colors are very nice and the tree shape makes me want to burst into tears (never very good at drawing trees so I'm jealous!). There are a few things though, that would make this even better.
Your use of color is great but on some sections of the piece looks like you didn't commit as much as others. The pants look perfect, just the right about of dark quality as there is lighter qualities. However, when you go up to the shirt, the dark quality is lost in a majority of the shirt (save for the sleeve on her left arm). I find that some works add "unnecessary" shadows sometimes just to highlight something (like having a very slight shadow on the string of her shirt to separate it from the rest of the white shirt). A very slight shadow can make things pop and give them dimension.
There also looks like there should be some more soft shadows around her breast area, very slight, and the neckline slightly more wobbly to show the curve from the chest and the arm. Other than those few things, the way the shirt falls is very appealing.
Her skin and body itself is very well done but there are a few things I noticed here as well. I think her eyelids could use a bit more work, giving them more shadow to a crease or something. Right now they look sort of flat on her otherwise nicely contoured face. Her neck is also looking a bit thick on the left side and might look slimmer if you showed a small section where the neck meets the shoulder. Just a thought. Her legs look slightly too think as they meet the ankle, more of a bony thickness that looks off and the leg further from us seems a little too lightly colored, the shading is wonderful so perhaps you could just darken that area all together. Her hand, though, is probably the worst part of this picture. It looks really jacket up in the last two fingers while the rest look very nice. Perhaps redoing those two slightly bent and showing more of the bent knuckles will ease the trouble of for-shorting at such an odd angle.
The only other thing I have to say was the ground covering was slightly disappointing, as well as her shadow on the ground and around the base of the tree. And if this wasn't a portfolio thing, I would suggest removing the simple one color grey background thing and making it transparent or at least a darker grey. Slap a few shadows and more grassy things and you should be good to go!
All-in-all the piece is great and Very beautiful. There isn't much wrong, I was just nitpicking mostly but someone of your skill, that's all there is to do when critiquing a piece like this! Hope this helps!
All your comments are spot on, I'm too tired to go back into it now! haha, but I do agree with you! I just need to work harder, and try to apply myself more fully to my work. I guess eventually you start becoming blind to certain elements and you don't realise it needs more/less work.