My mod doesn't work after going from 1.12.2 (plugin) to 1.15.2 in 2020.3

I own a mod called Loot Vases that is for 1.12.2, 1.14.4 and 1.15.2. The first couple of versions were small and in beta, then I made a huge version for 1.15.2 that was the official release out of beta. I than decided to backport all the new stuff to the older versions that my mod is available for. I switched the forge to 1.14.4, played it to check that it was stable, then I released it. Then, I downloaded the 1.12.2 plugin for MCreator 2020.3, and did the same thing with that. I did a few changes for the 1.12.2 release version, but it was mainly just texture and recipe changes to fit with the game version. I tested it and it was fine, so I released it. After it's release, I went back to MCreator to change the forge 1.15.2, as I'm not planning to do any of the older versions of the mod anymore as 1.12.2 and 1.14.4 now have official releases. When I tried to play the game, the console said that a bunch of things didn't exist, and it couldn't find symbols. I'm assuming the 1.12.2 plugin had something to do with it since this all started after I used it. Can anyone please help me out?