Thanks for reaching out. Just to confirm, have you tried changing your YouTube password with the steps on this page: https://goo.gl/nKme4X ? Once done, you'll be logged out of all the devices in which you are signed in. Let us know if that helps.
Thanks for confirming. After resetting your password, did you check all the email addresses you might've used to sign up or sign in to your account? If you do not recognize any email, you can remove it from there. More details here: https://goo.gl/3Y6vm8 . Hope this helps.