Disqus breaks theme in single posts

My website: https://www.edwardianpromenade.com

I added Disqus to the site and imported the comments, and everything works fine–except my custom theme. Both sidebars appear on the main page, but when you click on an actual post, the right/second sidebar disappears. I don’t feel confident with tinkering under the hood after spending all weekend fussing with the stylesheet, so if anyone would help, I would be most appreciative.

https://www.ads-software.com/extend/plugins/disqus-comment-system/