I've lost my portal home, and I am invincible. How can I leave the nether?

I've gone into the Nether and seemed to have lost my portal out of it, I also can't die in lava or by mobs attacking me, I'm appear to be invincible and can't die.

This is Single Player by the way.

Why is this happening?

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.

It sounds like you are in creative mode, then you are invincible. Can't you build a new portal?

If not; try to kill yourself by digging to the void, or below yourself...