Hello
I'm having a problem (aka. lack of knowledge)
I have Nginx as reverse proxy and Apache installed with wordpress
When I load a HTTPS page , some content loads as http .
What should i do to load all the content as https when I request an https page?
This is not nginx or even apache problem, is the code on your website, check all links and css, js files are being called using relative paths ie: /css/style.css (on your site code/php/html, not on the output you receive in your browser as this one will always be show as full path).
Also if you are calling external resources like js use //host.com/resource.js instead of http://host.com/resource.js so your site call it https or http when requested by the browser.
You could install a plugin from the plugin directory or WordPress HTTPS (SSL).
https://wordpress.org/plugins/wordpress-https/
https://wordpress.org/plugins/search.php?q=ssl
This may also help...
https://make.wordpress.org/support/user-manual/web-publishing/https-for-wordpress/