Overpower is your biggest friend

When I play with newer tanks as a regen-based healer, what I often find is that they will tag all of the enemies once (or worse, aggro them by running at them but not actually hit them with their enmity skills) and expect everyone to attack the main target together in an organised strike. That means that as soon as I start healing the tank (or an AoE job starts using area attacks), the aggro from that will rip the rest of the enemies straight off the tank and they'll run at me, causing a lot of stress for the tank (if they notice) or for me (if they don't).

When enemies appear mid-fight, they won't go on your aggro display until you have hit them with something which is a bit annoying. The healer will generally notice them right away because they'll passively aggro them just through healing - so you need to keep an eye on this and prepare to intercept. A couple of Overpowers should pick them up and then you can keep an eye on the additional enemies normally.

But in general, if people leave as soon as they hear you're new they're just jerks. Let them go. It's not your fault they have attitudes, and the fact that you're taking your performance so seriously shows that you have what it takes to be a really good tank.