I totally feel you, it took me 4 days on different computers until I gave in and got a new clean server just for lemmy. Then I used the Docker way of hosting. But even with that the documentation didn't tell me everything, so I had to change the second nginx config file (the one with server_name my_domain.tld;) so now this is working even with the web-workers (for posting comments) and federation with SSL:

map $http_upgrade $connection_upgrade {
    default upgrade;
    ''      close;
}
server {
    server_name jemmy.jeena.net;

    location / {
        proxy_pass http://localhost:8080;
        proxy_set_header    X-Forwarded-for $remote_addr;
	proxy_set_header Host $http_host;
	proxy_set_header X-Real-IP $remote_addr;
	proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection $connection_upgrade;
    }
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/jemmy.jeena.net/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/jemmy.jeena.net/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}
server {
    if ($host = jemmy.jeena.net) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

    listen 80;
    server_name jemmy.jeena.net;
    return 404; # managed by Certbot
}