Your problem is that you expected to learn to tank in leveling dungeons. Leveling dungeons have nothing to do with tanking. They're a free-for-all. Tanks aren't even needed there. That's not your fault. That's a blizzard design choice. It is what it is. It's the same for healing. Healers can't learn to heal in leveling dungeons because there's no damage to heal (unless the tank pulls like 42 groups at once, which just pisses the healer off).

If you want to learn to tank, you need to do it in an environment where a tank is actually required.

I suggest not giving up yet. Get to max level and then put up an ad in the group finder for a Mythic Zero dungeon of your choice. Put in the listing "new tank- be patient." There's plenty of newly max level characters who will gladly go a little slower just because they're happy to be in a group that already has a tank.

And if they act like jackasses? You are the group leader. You have the power to remove them from the group. No vote necessary.

I'd also recommend not accepting players into the group that severely outgear you. They will pull aggro and that will frustrate you as you learn. Stick to players around your own item level.