do you have an actual pinstriping brush? if not get one, it will make your life easier.
and of course, slow down and take your time. how do i know ya rushed it? cause ya didnt let the paint dry before ya moved on to the second color.
when i pinstripe i use two hands to paint. one to hold the brush and the other hand as a guide resting on the surface i'm painting. (paint brush hand resting on it.) i drag my guide hand over the surface as i paint. with such an intricate design you'll have to allow plenty of dry time or its gonna turn into finger paint.
if ya really want to hone your skills first start painting straight lines (as long as possible) once you can paint straight/even/fine lines, with your eyes closed, then move on to curved lines. once ya got the curved lines down, then try sticking the 2 together with more intricate patterns.
your design is very cool, sorta a tribial tattoo art kinda pinstripe, but the uneven, different thickness lines (ie the ones you painted a few times,) really take away from your design.
good luck.