How do I load this jQuery to my wordpress page?

I would download the jquery.mixitup.min.js and place it in the same folder as your custom script (js), also makes sense to place the “main.js” file in there also, keeping your js together, if you can.

Not sure if your using a child theme or not,if not but add the following to your functions file, overwriting your original code:

add_action( 'wp_enqueue_scripts', 'add_my_script' );
function add_my_script() {
wp_enqueue_script('jquery');
wp_enqueue_script( 'custom-script', get_template_directory_uri() . '/js/custom.js',array(), '1.0.0',false );
wp_enqueue_script( 'projects', get_template_directory_uri() . '/js/main.js',array(),false,false );

}