Inside the main content, add $page_id = get_the_ID();
.
And in the custom loop, call the fields:
the_field( 'main_page', $page_id );
get_field( 'main_page', $page_id );
Those functions work without an ID if used inside the loop, otherwise we need to specify what post we are requesting.
Also, you can filter the post type when calling WP_Query:
WP_Query ( 'post_type=post&category_name=' . $main_page );