Minecraft – I’ve lost the castle, how to find where I built it

It seems you are playing in creative mode, so you can easily make a new portal to go back to the overworld. If you don't want to, remember you can always double tap space to fly; that should make your hunt easier.

Even if you aren't, if you created your portal at spawn, the nether end of it should be in the general neighbourhood of X: 0 and Z: 0 (mostly within a 100 meters range of that). Press F3 to show your coordinates and try to head in a direction that brings both X and Z close to zero (Y is your altitude, confusingly). Keep in mind your portal might be high up enough that you can't see it.

You could just note down the items that you have got then kill yourself. Then, because you are in creative mode, you can give yourself the same items that you had before you died.

Just don't ever go back to the End.