Only show content before more tag

You can use the WordPress function get_extended to fetch the different parts of a string (the part before and the part after the <!--more--> tag). get_extended returns an array with three keys, of which the keys main and extended are important: $arr['main'] contains the part before the more tag, and $arr['extended'] the part after the more tag.

This would yield something like:

// Fetch post content
$content = get_post_field( 'post_content', get_the_ID() );

// Get content parts
$content_parts = get_extended( $content );

// Output part before <!--more--> tag
echo $content_parts['main'];