I have Nginx as a reverse proxy on port 80 and apache
I’m using wordpress , but while requesting an HTTPS page ,The page loads as https but All content such as (images , JavaScript , CSS ) loads as http (Without SSL)
I’m not sure how I can solve This problem
If you have successfully setup Nginx reverse proxy with SSL, then you should use a plugin in WordPress to make sure that it return https versions of the assets on your website. If there are hardcoded urls in functions.php, CSS or in plugins then you must manually change those to https.
I personally use SSL Insecure Content Fixer in my blog. If you use this plugin then make sure that HTTP_X_FORWARDED_SSL (e.g. reverse proxy) is selected in its configuration. If for some reason the plugin has trouble changing http to https, then as I said before you have to manually edit problematic urls.
I would also recommend you to read my article on how to setup reverse proxy Nginx with SSL for a website running on Apache.