The look in her eyes is what really caught my attention. The pose of her body and wings compliments her aura of thought, while the background and lighting does something phenomenal to this picture that I can't explain.
Only now do I realize that while I've seen this one, I haven't commented on it. Scatterbrain is bad for you.
I simply love the way she looks here. The lines of her body feel longer than in the show style, but comparatively the body size remains the same. Same goes for her wings. Her beautiful, beautiful wings. The way you depicted her feathers works well, both the long flight ones and the shorter ones on the inside. And I adore her mane. You reflected the varying strands and individual hairs beautifully, and the highlights look quite great indeed. Once more, I love the curves and lines of her body: the rear leg, the front legs, the curving back, the chest, the nearly-hidden neck, the somewhat more Celestia-like head and her lovely ears. Another big success of this picture is just how expressive you've made her: not just her face, but the entirety of her body speak of emotion. Here, it seems complicated: she's pensive, thinking about something that she is not sure about, and this shines through all of her. As for just the face itself, I really, really, really like the shading here, as it shapes her face beautifully (it does the same for all of her body, really, great job here!), and her eyes feel simply amazing. The somewhat longer horn goes in line with the Celestia-shaped head, and as for the ears... well, Twilight's ears are the most expressive ears in Equestria, as it has been proven multiple times. Finally, the background features nice transitions between the colours of sunset, twilight and early evening; the sparks liven up the atmosphere and give it a mystical air and the spiral of white either shining onto or emanating from her looks interesting and meaningful: perhaps she is finding consolation for whatever it is that troubles her? Regardless, looks great! Well done, well done indeed!
Twilight, my love, worry not. Everything will be fine. No matter what troubles you, my Princess, we will find a way and overcome it. No matter what. Remember this. When all of us friends work together, we can accomplish anything, and we will always support you both as our Twily and as our Princess. But tell me what's wrong, Twi... Perhaps it will help. At least it will help me: I can't bear the sight of a dejected and troubled you.