How to make this Form disappear once it performs its function

Although I know this isn't a state-of-the-art Log-In Form, it does function successfully.
If the wrong credentials are entered the user is re-directed to a log-in page.
If the correct credentials are entered the user stays on the homepage.
What can I add to this so that when the correct credentials are entered, this Form disappears,
until the user logs-out? Thanks for any help

<form action="../login.php" method="post" accept-charset="UTF-8" class="middletext">
<p>
<style type="text/css">
.form_label {
font-size: 12px;
color: #696969;
margin: 0px 0px 20px 0px;
}
#user_name_login {
width: 150px;
height: 38px;
border-style:solid;
border-width: 1px;
border-color:#e5e5e5;
}
#password_login {
width: 150px;
height: 38px;
border-style:solid;
border-width: 1px;
border-color:#e5e5e5;
}
</style>

<input type="text" size="20" name="user_name_login" id="user_name_login" value="ENTER USERNAME" style="color:#D9D9D9" style="vertical-align:middle"; onfocus="if (this.value=='ENTER USERNAME') {this.value=''; this.style.color='#696969';}" >
<input type="text" size="20" name="password_login" id="password_login" value="ENTER PASSWORD" style="color:#D9D9D9" style="vertical-align:middle"; onfocus="if (this.value=='ENTER PASSWORD') {this.value=''; this.style.color='#696969';}" >

<input type="hidden" name="cookie_time" value="10080" />

<img src="../themes/default/images/arrow-red.png" alt="" /><input type="submit" style="outline:grey" font-size="5px" value="[var.lang_login_now]" class="button-form2" />
<input type="hidden" name="submitted" value="yes" />
<input type="hidden" name="remember_me" value="remember_me" />
</p>
</form>

PHP:

 

That's... just... so... ugly.
Really. Inline javascript, CSS places haphazardly inside the <form> tag, value instead of placeholders, inline-CSS too...
I really, really, REALLY suggest you rewrite this.
For the hiding of the form, you can either do this by redirecting to another page (have the login as a separate page), or base it on a session or cookie and then hide the form based on that, or just add or remove an ID on body for loggedin/notloggedin and base the display of the form on that.

 

Thanks for your reply.
yes, I understand that the code can make modern coders cringe.
I didn't write the script, I just use it. I'm no coder, so all the things that you suggested sound good,
I just don't know how to code that.
I thought maybe I could get a line of code to head me in that direction.