Another quick tip I'll give to you is to use the smudge tool more often. It should be used just as much as the airbrush tool in fact most times. It would have helped you out alot here on this one

some very good comments above
use them to help you improve
but a nice design and style, good for a first brush,

Now try to look more at real designs to see how it should look
but keep it up mate

while were at it also look out with the smudge tool , on large surfaces it can do no real harm but when close to parts use a selection first then smudge

very quickie
a little toony
cya