what'd i do wrong?

[Bixby]

First off, don't take it too hard when someone gets in your face. Some people are overly aggressive. Some people especially have it in for tanks, some for healers, some for DPS. These same people are also often fairly clueless about the jobs they rag on. Don't let it get to you.

It's hard to say who's at fault here, since we don't know just what the healer was doing. It's not difficult for a healer or DPS to pull enmity if they're doing things wrong. It's also not difficult if the tank is doing things wrong. It sounds like you have a pretty decent grasp for the middle of the fight and onward, but you didn't go into what you were doing to open fights, so here goes.

Flash works for everything around you as long as it's in range. I've seen a lot of new tanks who seem to think Flash has a pretty large range; it doesn't. So be aware of that.

What I'd suggest is to start with Shield Lob, then once all the mobs have gathered, Flash at least as many times as there are mobs (e.g., 3 mobs = Flash Flash Flash before you start your rotation). Then do your threat rotation on the main target once. Then start spreading your single-target hits around. Fast Blade mob #1, Savage Blade mob #2, Fast blade mob #3, Savage Blade #1, Fast Blade #2, Savage Blade #3, and so on. If something breaks away, Shield Lob it and then either Flash or threat combo it.

In another level, when you get Provoke, use Provoke > Shield Lob to get something back. Provoke gives you 1 more enmity than the person with the highest enmity, so it 1) doesn't do you any good if you already have enmity and 2) isn't enough to keep the mob's attention without a follow-up, but it does make sure you get the mob off whoever it's on with one move (and then you have to keep it off them).

If you feel more comfortable weaving Flashes into your rotation, go ahead as long as you've got the MP to support it, but starting with a few before you start your single-target rotation is a good idea.

The stun on Shield Bash can prevent a little damage, but it's not a high-threat move, so it's not so great for getting something to get back on you.

Also, don't save your cooldowns for boss fights. The recasts on most of them let you have them up for most trash and still have them ready for the boss, too. Using defensive CDs on trash packs mean less healing, which means less enmity for the healer.

[Nahara]

If you feel more comfortable weaving Flashes into your rotation, go ahead as long as you've got the MP to support it

As a follow-up to this, the Riot Blade combo is a great way to help get your MP back up if you're weaving Flashes into your normal rotation. Just don't use it too often, as you will be wanting to do your main threat combo on everything most of the time.