Our site has lost substantial traffic after moving to HTTPS.
Keyword here is moving.
If you just changed everything over to HTTPS from HTTP (and got rid of the HTTP version), then many robots that had access to your site before the change will think you don't have a site anymore since they will come across a series of "404 not found" based pages generated from your server which is bad news.
What you need to do is create redirects for every HTTP page you previously made available to the public so that users who try to access the old pages will be automatically redirected to the new pages. To ensure the redirect is perfect, ensure the HTTP header response for the old page contains HTTP code 301 (moved permanently) and that a location header exists pointing to the HTTPS version of the same page. Redbot.org is a good tool to verify that your headers are correct.
Should we go back to our last without HTTPS version?
If you do that, you might throw people off all the more because they may have one version in their bookmarks and if you switch back again with no regards to implementing redirects, then you'll frustrate the new batch of people since they will get the dreaded 404 error.