Your first config (with Let's Encrypt cert

server {
    listen [::]:80;
    server_name mydomain.com mydomain.com;

    return 301 https://$host$request_url;
}

server {
    listen [::]:443 ssl;
    server_name mydomain.com mydomain.com;

    root /var/www/ghost;
    index index.html;

    ssl_certificate=/etc/letsencrypt/live/bishesna/fullchain.pem;
    ssl_certificate_key=/etc/letsencrypt/live/bishesna/privkey.pem;

    location / {
        try_files $uri $uri/ =404;
    }
}

Your second config (with self-signed cert)

server {
    listen 80;
    server_name ghost2.mywebsite.com www.ghost2.mywebsite.com;

    return 301 https://$host$request_uri;
}

server {
    listen 443 ssl;
    server_name ghost2.mywebsite.com www.ghost2.mywebsite.com;

    ssl_certificate /etc/ssl/certs/selfsigned.crt;
    ssl_certificate_key /etc/ssl/private/selfsigned.key;

    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers HIGH:!aNULL:!MD5;

    root /var/www/ghost2;
    index index.html;

    location / {
        try_files $uri $uri/ =404;
    }
}