what'd i do wrong?

[AkikoPanda1]

You're not entirely at fault. The white mage chose not to/didn't know how to teach you what you could do differently, and instead decided to be a jerk.

You don't have to target a specific mob to make Flash work. The hate in this game works differently than in other games; in WoW, for example, you only needed to hit a mob once to keep aggro on it, as long as no one else hit it. That is not the case here. The healer still generates hate as he or she heals, so you need to spread your enmity generation around the multiple mobs you may be tanking. I would recommend cycling through the mobs you're attacking with your skill chains; the enmity bar can be helpful with this.

Let's say you're fighting 3 mobs: A, B, and C. Once you hit them all, you should get the little vertical bar that displays their name, health, and enmity status against you. The enmity is displayed as a shape on the left-hand side. A red square means you have the most hate from the target, an orange triangle means you have less, a yellow triangle even less, and a green circle means you have almost no enmity against it. I keep this in a convenient location, in case my tab-targeting jumps to something I don't want or to click on a mob that switches targets. The only downside to it is that by the time the shape changes, it means someone else has pulled hate from you. However, there is another place you can look for enmity status. When you have a mob targeted, you can look at your party frames and see a white progress bar under the class symbol. This is the percentage of enmity that person has against the mob; when the bar is full, the mob will switch to that person. The person with the highest enmity against a target will have an "A" next to the bar.

So you start your combo on mob A, then tab to mob B and use the next skill in your chain, then the same with mob C. I keep one eye on the enmity list so I know I'm targeting a mob I want and not one on the other side of the room or just out of my line of sight.

Are you also using defensive cooldowns? It would ease the burden on the healer, which reduces the amount of hate he or she generates, making it less likely for him or her to pull hate from you.