MacOS or Windows? I don't remember where the old FTB launcher put things on the Mac, maybe in a folder in the users/library/ tree? The new launcher should be putting them in the Documents folder in a folder labeled Twitch or Curse, unless you specified a different location on install. On Windows I have no clue. A file search for whatever you named your world when you created it should find it.
Assuming you are a Windows user and are not hugely technically competent, download and install Everything
Everything is a simple search tool that uses your hard disks own indexing to instantly search, well, everything.
You can use it to find, with no knowledge of where twitch / classic / FTB / MultiMC etc. typically store things, where your various Minecraft profile folders are. You could use your regular hard drive search, but its slow, oh so slow.
All Minecraft launchers store their instances as a very similar Minecraft profile folder, typically the instance folder contains a sub folder called ".minecraft" or just "minecraft", and that folder contains the various Minecraft folders that make up a typical profile: "mods", "config" etc. and most crucially "saves". Use any of these folder names as key phrases in Everything and you will quickly find where Twitch or FTB or whatever has stored the saves for your various instances and then be able to copy the save you want.