Well first, nobody said the CP had to win, just to extend the fighting for as long as possible; even if they were still doomed, it wasn't written that things had to fall apart starting when it did or as quickly as it did. I already mentioned the Vardar Offensive (and Ludendorff's subsequent breakdown), the Second Paris Conference, and the Naval Rebellion. Second, the CP did get a kind of second win with Brest-Litovsk, which opened up not only troops from the Eastern Front, but resources from the Ukraine, etc. Third, a big part of how the Allies managed to fight on in the Spring and Summer of 1918 was logistics (getting food to Paris, coal to Italy, etc); if, for example, the Spring Offensive had managed to capture the rail lines at Amiens and cut the lines from Paris to Northern France, then said logistics would be interrupted to the point that not only France but Italy would find themselves in situations even more politically precarious than OTL.