Is there RELATIVE_PATH, like ABSPATH

You can use str_replace with __FILE__ to get the location of the current file.

str_replace(ABSPATH, '', __FILE__);

So for example if __FILE__ returns:

/users/chin/public_html/project-name/wp-content/theme/theme-name/functions.php

The output would be:

/wp-content/theme/theme-name/functions.php