Custom "wp_login.php?action=lostpassword"

hello,

I’ve got a custom registration form which works. Then, on this form, I have the “forget password” link. This one is not working very well. The “forget password” form is OK with its design; enter a login or email address to receive an email and to change the password.

I have 2 cases :

If the login is empty or false
If the login is right

I have the following code :

<?php>
//traitement des erreurs de connexion
$errorlost2 = false;
if ( !empty( $_POST ) ) {
    $user_login = $_POST['user_login'];
    $user = username_exists( $user_login );
    echo 'user_login =' .$user_login; // renvoi bien le login
    echo 'user =' .$user; // renvoi bien l'ID du user si login exist
    if ( $user == NULL ) {
        header( 'location:lostpassword' );
        $errorlost2 = true;
    } else {
    }
}?>
    <div id="check-lostpassord" >
    <div class="titre">
        <h1> <img src="<?php echo $cheminsite; ?>/images/lion.png" alt="lion"> <?php echo $lang['TITRE-CONNEXION']; ?>      </h1>
        <div class="titre-underline"> </div>
   </div> 

   <div class="connexion-lostpassword">
    <div id="box-lostpassword">
            <p class="message"><?php echo $lang['lostpassword_message']; ?></p>
            </br>
                <div class="error">
                    <p id="lostpasswordform-erreur2">
                        <?php
                        if ($errorlost2):
                        echo 'TEST';
                            echo $lang['lostpassword_error2'];
                        endif
                        ?>
                    </p>
                </div>

            <!-- http://club-d-affaires.de/wp-login.php?action=lostpassword -->
            <form name="lostpasswordform" class="lostpasswordform" action="<?php echo site_url('wp-login.php?action=lostpassword', 'login_post') ?>" method="post" id="lostpasswordform" >

        <br class="clear"></br>
          <p id="text-form">
            <label for="user_login" ><?php echo $lang['lostpassword_login']; ?>*:<br />
            <br class="clear"></br>
            <input type="text" name="user_login" id="user_login" class="input" value="" size="20" /></label>
          </p>
            <br class="clear"></br>
            <br class="clear"></br>
           <?php do_action('login_form', 'resetpass'); ?>
            <p class="submit">
            <input type="submit" name="user-submit" id="wp-submit" class="user-submit" value="<?php echo $lang['lostpassword_envoyer']; ?>" tabindex="1002"/>
            <?php $reset = $_GET['reset']; if($reset == true) {?> </br></br> <p> <?php echo $lang['lostpassword_confirm'];?></p>
            <a href="<?php "http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']?>connexion/?langue=<?php echo $langue; ?>" >
            <h2> <?php  echo $lang['lostpassword_connect']; ?> </h2>    </a>        <?php } ?>
            <input type="hidden" name="redirect_to" value="<?php echo $_SERVER['REQUEST_URI']; ?>?reset=true" />
            <input type="hidden" name="user-cookie" value="1" />
            </p>
        </form>

       </div> <!-- fin box-connexion -->
    </div> <!-- fin connexion -->
</div> <!-- fin connexion -->

The problem is at the beginning of <form> :

Point 1 – Error message, if login is false or empty, works if I have :

<form name="lostpasswordform" class="lostpasswordform" action="" method="post" id="lostpasswordform">

Point 2 – Send an email, with the link to modify the password, works if I have :
<form name="lostpasswordform" class="lostpasswordform" action="<?php echo site_url('wp-login.php?action=lostpassword', 'login_post') ?>" method="post" id="lostpasswordform" >

The action=”” is the problem here. How can I have point 1 and point 2 working at the same time?

Many thanks for your help