Chris on "trying processing data form"

You already seem to have registered an admin menu page (where the form will be shown). So you can take this very page URL (and script), use this one as form action value and include the process.php there.

<?php
if( {checkIfFormHasBeenSent} ) {

  include_once dirname( __FILE__ ) . DIRECTORY_SEPARATOR . 'process.php'

}
?>
<form action="admin.php?page=whateverYourPageSlugAlreadyIs" ...

Or if you really must target your process.php directly you could do it like this
<form action="<?php echo plugins_url(); ?>/yourPlugin/process.php" ...