First, understand that as a tank you demand space. You are a very presence on the battlefield tells everyone: If you want to get to them you got to go to me, so it stands to reason that positioning is everything. If you're a Reinhardt holding your shield but your whole team is in front of you well that's just not going to fly; sure you may feel that your team has pushed up too far and risk being taken out but what's the better option here. Choosing the better position further back while your team gets picked off leaving you as the last man standing or just going in against your better judgment just to ensure your team has a chance in that fight.
So the first thing you have to understand is that tanks are there to create space. Why is this important? Well, it's simple, it gives room for your DPS to do their job without interruption, so if you're creating space but have bad positioning, it doesn't benefit anyone. Your priority as a tank is being a distraction, not securing the allies. I know it could be a bit strange since all the tanks have weapons for dealing damage but arms aren't for getting kills necessarily aside from Roadhog, but he's a particular case. Yes, I understand that tanks like Winston and D'va have weapons and abilities ideal for chasing down and taking out a target that's out of position or low on health. I'm not saying securing allies or deletions if you prefer is not on your list of priorities at all. What I'm saying is that it's not at the top of your priority list, it's more like number two or three. Your priority is getting the enemy to pay attention to you being a distraction. I mean if you're a Mercy player that I know you can relate to this but how many times has a Tracer or a Genji just walked right by your tank like a Reinhardt heading straight for you while he just stood and held the shield. Sure it's not necessarily his job to deal with them. That's really a DPS job, but he's not demanding the space a tank deserves and therefore the DPS may have trouble protecting you, and of course there's always exceptions, but in general, as a tank, you have to be a distraction and command the presence that you deserve.
As a tank, you have to learn to let things go what do I mean by this I mean that you will be at that moment where the mercy is by herself, low health and you know you can finish her off, right? You know you can, she's just right there, so you go for it. Except your damage output isn't that ideal being a tank and also you just can't quite finish her off, and she's running away and getting away the pressure is on you get tunnel vision she becomes that carrot on a stick that's just out of reach.
Overwatch tank guide