Complex sentence: Despite his celebration of "The Monk", Sade believed that there was not a single Gothic novel that had been able to overcome these problems, and that a Gothic novel that did would be universally regarded for its excellence in fiction.
Split into simpler sentences: Despite his celebration of "The Monk", Sade believed that there was not a single Gothic novel that had been able to overcome these problems. He theorized that if these problems were successfully avoided within the genre that the resulting work would be universally regarded for its excellence in fiction.