Change used graphics card in Minecraft?

Minecraft has not been using my better graphics card for a few days, but the integrated one in the CPU. If I'm in the NVIDIA Control Panel and then change it, the change only takes effect in the Minecraft Launcher and not in the actual game.

Are you using the Java version?

Then you have to make the change in the NVIDIA Control Panel for the entire Java Runtime.

@Mourlie Where can I find this "Java Runtime"?

You will likely need to add them to the list in the NVIDIA Control Panel first (javaw.exe)
Unfortunately, I can't tell you where it is installed in a hurry.