Buttersvtch

Hello, I tried to get my mods separated for 1.12.2 and 1.16.5. Now they're separated in their correct folders, but as soon as I log into the game they won't load. I am able to log into the game and make worlds, but when I create a new world with any of the mods, the mods will not load into the world at all. Even though on the screen that says "single player, multi-player, etc." It says the version of forge, minecraft, MCP, and the number of mods loaded. How can I fix this? I'm terrible with this sort of thing