Widget only in first post

Assuming you are trying to add content only to the first post in the loop, a static variable should do it:

function pietergoosen_insert_content_filter( $content ) {
    static $first;
    $first = (!isset($first)) ? true : false;
    if (true == $first) {
      ob_start();
      $sidebar = dynamic_sidebar('sidebar-19');
      $new_content = ob_get_clean();
      if ( ! is_admin() ) {
          return pietergoosen_insert_content( $new_content, 2, $content );
      }
    }
    return $content;
}
add_filter('the_content','pietergoosen_insert_content_filter');