Its a shame Paladins have to be tanks. I understand.

[Miiu]

After playing this game for around a decade I either was extremely lucky or it's an attitude thing but I never had any real toxicity in dungeons then maybe a handful of instances. But that usually was someone being just an idiot and directing their anger towards everyone.
It took a while for me to pick up tanking. I don't think I started doing so before the end of heavensward but because I mainly healed I was familiar with pull patterns in most dungeons. Nonetheless did I stick to smaller pulls when doing a dungeon for the first time as tank and every time I'd say so in chat beforehand.
"Hey I am new to tanking. Is it okay if I take it slow this run?" And I never got any shit for it. And that is still true today. A friend of mine recently started this game and I don't recall a single instance where telling the group that he is new and is still trying to get a hang of things has turned into critique. Either the rest of the group stays silent and just does their thing. Or you get people who are more familiar with tanking giving advice.

But obviously this is a big community so I am not dismissing that there is toxicity in the game. I am sure there are people who are being harassed for being new. But that is definitely not the norm.
So my advice would be to be open about your nervousness when tanking and that you need to go at a slower pace than people might be used to.