The lighting is phenomenal. Her expression is well done.
It seems to me that the shading on her neck is too symmetrical, at least considering how her mane only falls on one side. Speaking of her hair, it should part around her horn. Right now, it looks like her horn is just phasing through it.
One last note: I don't like the way the tear just starting to roll down her cheek is drawn, but I can't quite pin down why. It just looks off. I wish I could be more specific.
On the whole, this is an excellent piece which obviously required skill and passion. It just has room for improvement, as all art does. Not sure if a critique this long after you drew the piece is still helpful, but maybe it's a reminder that people are still discovering and enjoying your art.