Sorry to say. But as a a player who main Tank if you blame entirely on tanks. You either 1)Main a DPS or 2) Have not tanked enough 3) Wasn't severally overgeared and 4) Never met DPS who can generate extremely high aggro. AKA good dps.
I don't want to point fingers but please don't self proclaim yourself as a good tank when you obviously didn't even go into high level contents with tank before (From Achievements point of view)
Now lets put the blame aside and help you improve your game play rather than point fingers.
Firstly gain more experience. Think more about what you can improve rather than pointing fingers and expect others to get better. Because they won't if they are pointing fingers themselves
What you can do.
1) Use 1 or 2 more AOEs as starter
2) Use your CDs like fight or flight for PLD Unchain/ Berserk for WAR. I know you don't always have them up but it will improve your overall performance
3) Split your combo. Example. You marked mob 1. One dps is hitting mob 1 the other is hitting mob 2. And that DPS is a melee dps who can generate high burst aggro from start. Don't panic. Take your time to get the aggro back.
How to do it. there are many different ways. Since you open on mob one. You obviously have more aggro on mob 1 so after doing one full combo on mob one, start comboing on mob 2 and alternate it. You play around with how you split your combo, I'm just giving an example. 2nd way is to do combo 1 and 2 then provoke > combo 3 since combo 3 have the most aggro. 3rd way is if the 1st mob is low on health like 30% or less, feel free to switch to the 2nd mob since even if you lose aggro on the first mob, it will only be for about a few seconds. There are many ways to do it. You can experiment and play around with it.
4) Start next pull early. When you are hitting the last mob till about 15% of the HP. Feel free to either stun it or let ranged dps finish it. There's no need to stay around hitting it because your job is to tank well. If you start the next pull early, you can use one or 2 more aoes or even split your combos from start.
There are many more ways these are some of my own. My point is if you are new. Have a open mind and accept that you are bad. Since no one is good from the start especially when Tanks have a Wider learning curve than DPS. Don't care about those butt hurt self proclaim pros but i won't blame them since any people who main dps will fire back when insulted.