In Daggerfall I found a Daedric longsword. (My char is like level 12 I think). I was like "Alright! Now killing enemies will be easy!" But, it seemed as though the enemies got stronger, so having a better weapon had no effect at all.

Does finding a really good wepon/ piece of armor in Daggerfall make all the enemies stronger?

If having a good wepon/ piece of armor really does make enemies stronger, when I sell or throw away that item, will it make the enemies go back to "normal?"

If your Long Blade skill is decently high, killing "should" have gotten easier. (Have you been using a Long Blade all along)?
Some enemies will get stronger, but that's due to leveling up, not the weapon you're using. Throwing it away won't do any good.

Long Blade is the only weapon I use... Weird.
I guess I was just imagining things.

There is level-scaling in Daggerfall.

At Level 12, you can already one hit kill most of the low level monsters, like harpies and spiders.

If your Long Blade skill is decently high, killing "should" have gotten easier.