load form via ajax and submit by ajax not working

I want to load a form via ajax and submit it by ajax.
But it not working.
my code:
【It is a wrong way to wast my so lost of time!!!!!!!!!! I hope someone else not do this through this way,so many unknowed problem】

1.add the library

function promycustom_preprocess_page(&$variables, $hook) {
  drupal_add_library('system', 'drupal.ajax');
  drupal_add_library('system', 'drupal.form');
}

2.hook menu

function mycustom_menu(){
$items=array();
$items['test/page']=array(
          'title'=>'ajax test',
          'description'=>'this is the ajax test',
          'page callback'=>'mycustom_ajax_test',
          'access callback'=>TRUE,
          'type'=>MENU_CALLBACK,
          );
return $items;
}

3.page call back,load the form and drupal setting

function mycustom_ajax_test(){
   $form= render(drupal_get_form('mycustom_change_user_brief_form'));
 
  $settings = FALSE;
  $javascript = drupal_add_js(NULL, NULL);
  if(isset($javascript['settings'], $javascript['settings']['data']))
  {
    $settings = '<script type="text/javascript">jQuery.extend(Drupal.settings, ';
    $settings .= drupal_json_encode(call_user_func_array('array_merge_recursive', $javascript['settings']['data']));
    $settings .=  ');</script>';
  } 
  return $form . $settings;
}

4.form build

function mycustom_change_user_brief_form($form, &$form_state){
  $form['abgegw']=array(
      '#markup'=>'welcome load ajax',
    );

  $form['submit'] = array(
     '#type' => 'submit',
    '#value' => 'change brief',
    '#weight' => 20,     
     '#ajax'=>array(
        'callback'=>'ajax_user_brief_handler',
        'wrapper'=>'content',
        'method'=>'html',
        'effect'=>'fade',
      ),
   );
  return $form;
}

5.my simple ajax call back

function ajax_user_brief_handler($form, &$form_state){
  $output='<div id="user-brief"> ajax handler success</div>'; 
  return $output;
}

6.in any other page load the form via ajax
the click button in any other page

the js code

(function ($) {
Drupal.behaviors.mycustom = {
attach: function () {
$(".ajax-load").click(function(){
("#screen").load('/test/page #mycustom-change-user-brief-form',function(){Drupal.attachBehaviors();});
});
});
};
})(jQuery);

when visit the page /test/page, the ajax submit work well.
But when load the form via ajax in any other page,it only can click to load the form via ajax,but can't submit by ajax, when click the submit button it will go to the /test/page,
I have search for long time to find how to solve it ,but still can work it out.
How to solve it???any help will be full of thanks!!!!!!!