Judging by the illustration that you posted I have to say 2 things:
1) If you decide to quit, the world will be a poorer place because of it. You have such a great understanding of anatomy, values, palette, OMG you even used a texture to polish your drawing which shows you don't half-ass things. This is the kind of quality art the world needs, which means the world needs YOU. So do mankind a favor and get back into the amazing journey of art, for all our sakes!
2) Everybody feels down and untalented at times. It's absolutely normal and impossible to prevent that from happening. The secret is to keep going forward! Hey if Brad Rigney calls himself a hack, what should we call ourselves? XD
Failure will keep you humble, and through humility you will stay level-headed and focused. And that's a good thing!
I leave you with this bit of the magnificent Karla Ortiz. I watch it every once in a while to get perspective.
Good luck and keep drawing!