Have a new form for each row...
<form method="get">
</form>
Like this:
foreach($portfolio as $portfolio){
echo "<tr class ='table-comments'>";
echo "<td>".$portfolio['portfolio_title']."</td>";
echo "<td class = 'comment-content'>".$portfolio['portfolio_client']."</td>";
echo "<td><a target = '_blank' href = ".$portfolio['portfolio_link'].">".$portfolio['portfolio_link']."</a></td>";
echo "<td>";
echo '<form method="get">';
echo "<input type='hidden' name='portfolio_id' value='" . $portfolio['portfolio_id'] . "' />";
echo "<input type = 'submit' value = 'Edit'>";
echo "<input type = 'submit' value = 'Move to Trash' class = 'action-button'>";
echo "</form>";
echo "</td>";
echo "</tr>";
}