How do I brush up on my art? My drawings look flat :'(?

Your art is an expression of you and who you are, not for anyone else so if you want to get better spend more time practicing the way you want it to look, sometimes i even draw out the same thing like 20x. Excellent work though i like your style.

There’s no modelling on the face.  You’ve not shaded and given form to the features that creates the illusion of depth.

Stop drawing on flat paper.?

At least you've gotten past the coloring book outlines.  Go lighter on the face framing,  darken the hair,  remove that odd line on the cheek (it doesn't coincide with bone structure or cheek shape). Fix the neck, it doesn't look like a neck especially with the bottom line marking off what looks like a stand for the head.  Leave some lines open. 

Nose, lips and chin are great.  Eyes are a bit exaggerated. Not sure why one eyebrow merges with the nose.

You are already beginning to introduce different weight in your line work, make more choices about what should be thick and what should be fine, looks like you are drawing on a tinted background, add white or lighter colours. You have started to identify where there is dark (almost always around the eyes where they are shaded by the brows), exaggerate that.

Take drawing classes

work on your shading and perspective techniques. Which direction is the light coming from? make it look like it. Better yet take some art or drawing classes. Have you thought about an art degree? how about graphic design?