We'd love to help. You can remove a custom URL from your channel up to 3 times per year. Here's more info on how to remove custom URL & claim a new one: https://goo.gl/bs9P2Y . Let us know how that goes.
I did, but I ended up deleting my YouTube channel instead. I just got a 2nd channel with no custom URL instead. I lost all my video uploads, but it's fine. I have the originals, so it's no loss. Thank you anyway.