all of a sudden, I can't kill goblins anymore, let alone fire atronarches, trolls and ogres. Is it true that when you continue to level up, your enemies get more and more difficult to beat?
Some creatures in Oblivion are scaled, and some are not. Rats, Wolves, Slaughterfish, Mountain Lions, Trolls, Bears, Boars, Clannfear, Zombies, Wraiths, Spriggans and Will-o-the-Wisps (to cite just a few) are never scaled in Oblivion. These creatures are always static (i.e. they remain at a fixed level no matter what level the player is).
Of the four creatures you mention in your post two creatures (Troll and Flame Atronach ) are never scaled. Flame Atronachs are always level 7, Trolls always level 6. Ogres do scale, however. And in the case of Goblins, it depends very much which Goblin you're talking about. Anything you see labeled "Goblin" in your game does not scale - it will always be a level 1 creature. Also, Goblin Skirmisher (level 4), Goblin Netherboss (level 4), Goblin Berserker (level 8) and Goblin Ambusher (level 14) never scale. Goblin Shamans and Goblin Warlords do scale. If you're having trouble with Goblins my guess is that it may be a Shaman or Warlord.
Many players dislike this aspect of Oblivion. Without knowing the specifics of your character or how you are playing, it's difficult to give you advice about what you could do differently to make your game easier.