As Nothingismagick says make sure you're drawing what you see not what you mind says the object is. And make sure when you "see" something you're really looking at it. A simple example - a plate is round, that's what my mind tells me. But more often than not when you draw a plate you're drawing an ellipse, not a circle.
The eye you drew - the white is very seldom pure white. Think of the location and shape of the eye - the eye is under the brow, recessed so often the white part is shaded somewhat, and since it's a sphere as you approach the corners of the eye they are going deeper below that brow and getting slightly, ever so slightly darker. And what are the proportions of pupil to iris to white area showing in respect to each other. You start to understand that by observing, really looking at your subject.
Draw loosely at the start not committing to hard lines - it will look sloppy but you can always erase what you don't want. I'd use (and I do) simple lead pencils to sketch out drawings. Whether they are for paintings or just a sketch on it's own.
Draw simple items to start - human figures or even parts of the body are by no means a simple drawing exercise. Simple still lifes are good starts - to this day I will find myself drawing an apple just to contemplate the shape and how to define the form with shading.
When you want to start drawing human figures/parts search online for the masters and how they sketched items - Da Vinci, Michelangelo it's amazing how they represented figures with simple lines, very few lines in some cases. Study what they do to represent an eye. And always notice how form comes from shading - variations of a hue if you're using color.
Also do you have any formal training? There are tons of tutorials online to help beginners and advanced folks - take advantage of them. And if you draw something and don't like it try to figure out exactly why you don't like it - then throw it away and try again. I can literally sketch the same subject 50 times before I approach what I want.
I've been drawing a long time and hopefully my suggestions help. Good luck and keep trying it will come.