I don’t know why login broke with force https if your SSL certificate was set up properly.
Have you tried safe mode?
example.com/safe-mode
or login backdoor?
example.com/u/admin-login
Given that none of the above work for you, If you’re familiar with SSH you can log into your server via ssh and follow these:
cd /var/discourse
./launcher enter app
rails c
SiteSetting.force_https = false