Where did the Nightmare and Hell difficulties go in Diablo 3 after the expansion?

I recently felt like playing Diablo 3 again, which I bought at release and played for a month and then lost interest. I realized they changed a lot of stuff, but I cannot make sense of those difficulty levels. I can choose to switch a game to "hard" or more, and there is some advice about having good gear and stuff. But I cannot understand where Nightmare and so on went.
I created a hard game with my level 59 sorcerer, went into act 2 and faced mobs with 1M hitpoints, which pretty much killed me instantly. Then I tried normal, and again it was a very hard fight. The items those mobs dropped were something like 100%+ upgrades to my gear.
With this character I used to play in Nightmare before.
I don't want to read two years worth of changelogs, so my question is simple: Where did the Nightmare, Hell , etc, selection go, or how was the difficulty scaling changed between release and version 2.2?
Best Answer
To sum up the changes: The old difficulty system "master one level and advance to the next" is gone completely. You can now choose your difficulty settings from the beginning. The higher the level the more rewards you can earn, but the nastier are the enemies.
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How do you unlock Nightmare mode in Diablo 3?
The 4-tier difficulty level is tied to progression throughout the game. To unlock "Nightmare" (the 2nd difficulty), you must first complete act IV on "Normal" difficulty. Nightmare is balanced based on the level you would be by beating through Normal, so it's not feasible to just skip ahead, even if you could.What is Inferno difficulty in Diablo 3?
Inferno mode is the fourth and highest difficulty in Diablo III. This mode is for the most die hard Diablo players who want an incredible challenge throughout the entire game. To access Inferno mode one must finish the game on Normal, Nightmare and Hell difficulty levels.How much harder is nightmare than normal Diablo 2?
Nightmare Difficulty in Diablo 2 ResurrectedThe Nightmare Difficulty is the exact same game, with some modifications since the level is now no longer on the basic difficulty. You'll face stronger enemies with increased health that deal more damage among other increased challenging situations.How do I get to nightmare in Diablo 2?
For having access to the Nightmare difficulty you must meet one of the following requirements:Diablo 3- Upgrading Staff of Herding to nightmare and beyond
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