Most of people love double eyelids, and lots of girls apply cosmetic surgery for double eyelids. So you need to cherish your god given double eyelids.
But I do know how hard it is to use eyeliner because I have double eyelids as well.
I was frustrated at the very beginning while the eyeliner I paint was covered by my extra eyelid, I bet you have this problem.
Later I quit using eyeliner since I found out if I use eyelash extension more than usual can lead to perfect eyes.
It's worth trying.
We most people, I think will, hold the view that double eyelids are more beautiful than single eyelids, so they even go to get plastic surgery to have double eyelids. So you must look better with double eyelids. Just keep your double eyelids and take more time to use eyeliner, that's my opinion.