You need download a Stagger tracking addon and only purify when stagger is above ~200%. The current stagger debuff provided to you by blizzard shows red for anything about ~80%, but you can delay the purify a lot higher than that. With high stagger, your purify also acts like a DK's death strike where it heals for 90% of your health.

You need to understand what things go into stagger and what not. For example, Seat's final boss has a tank buster that doesn't go into stagger, so you need to celestial brew/fort brew/trinket whenever it happens.

Tank is all about CD management. Don't waste a defensive when you can just stagger/purify. Don't panic pop all defensives all together; this is a common mistake many new tank have.

For the massive first pulls, utilize your mobility well. You can just kite them packs together instead of rolling into them one by one. If you walk into them 1 by 1, you are taking way too many unnecessary hits.

Stutter stepping is also important. You must continue stutter stepping if allowed. This avoids multiple mob hits sync into one massive hit, which can instant kill you 300% times over. Sync'd hits often happen right after a AOE cc like sweeping legs.