Combining similar shortcode functions?

I don’t know exactly what you are trying to do but perhaps something like this:

function customfields_shortcode( $atts ) {
  if (!empty($atts['field']) && get_post_meta( get_the_ID(), $atts['field'], true ) ) {
    $customfield = get_post_meta( get_the_ID(), $atts['field'], true );
    return "custom field >> $customfield << custom field";
  }
}
add_shortcode( 'mycustomfields', 'customfields_shortcode' );

echo do_shortcode('[mycustomfields field="customfield_1"]');

One shortcode will handle all of your custom fields. Just pass in the field you want as an attribute (argument).