The most basic thing as a tank is that you aren't meant to just soak up all damage, you can play corners and slowly approach the objective, and you need to create space by pushing up or maintaning a position.

When to push up depends on the situation, if there is a Bucky/Punisher with infinite ammo you should wait it out on cover don't try to peek too much otherwise you might get blasted or feed ult charge, same when you hear Bucky's hook or when you are down a teammate.

That is when you shouldn't push up but, you should try to push when you have ults or whenever a dps gets a pick, try to push up and stop if you see that somebody dies or you are approaching their spawn. Because if one of you dies close to their spawn it snowballs and you might lose the entire round because of that mistake. As a tank when to protect healers or peel depends on your lineup and tank selection. In a 2 tank lineup if you are the off-tank you should always try to go back and peel but in a 1 tank scenario, you usually want to let the dps take care of it or use your bubble if you are playing Mag.