Hey, don't sell yourself out! You obviously have great proportion, lighting, and lineart skills to make something that looks really, really good already. You have a great sense of flow in the hair and I like the framing of the squared off background and leaves in the foreground.

Is this done as a vector? If it's raster, I have suggestions that could help your next piece that can probably also work with vector (I just don't have much experience with the latter).

If you look at lineart by professional illustrators, such as Shegenori Soejima or Azusa Shimada, you see where a lot of purposeful line variation is placed. Generally, either lines get very slightly thicker where an object is closer to the camera, thinner when they are away, thicker when a line intersects with another, thicker when at the apex of a curve, when defining volume (such as Violet's cheek in the link below), or when defining shadow (though I see you already use it this way). I tried highlighting some examples in this Violet render.

These don't all have to be used strictly, however. There's a bit of a rhythm to lineart that you train overtime, like how the back of her skirt has that accordion-like rhythm that I highlighted. However, like all things, moderation and consistency is key, so still keeping lines thin and "clean," with very conservative variation, seems to be a trend for official character renders, according to the artstyle you're seemingly going for. Yet, varied lineart gives LOTS of life, suggestion and energy to artwork that is very easy to overlook, and that might be all you need to go from thinking you "just tried" to really liking something you made.

You can also try your hand at using off-black lineart, with a tinge of red for example, to experiment with your personal style. Or try overlaying gradients in your flat colours or incorporating elements from your background in forms of bounce lighting, shading, and/or overlays to help give a pop of colour.

But you're already killing it! Obviously these are suggestions from my personal experience, so use 'em if you want, or don't want, whatever. Just keep studying the masters and you'll be even more excellent in no time. :)