Steve shuts his eyes. He knows he hurt him. It's impossible to escape that fact. He has to give him something or risk making it worse when they just started to get to a good place.
He lets out a pained sighs and explains, "Natasha almost bled to death right in front of me because he shot her. He threw another friend off the Hellicarrier. And I... it wasn't good for me either. When I say I don't want you to help what I mean is I don't want to you to die." That is the likely scenario going in. He could lash or be under someone's thumb again. He can't let Tony be killed for his mistakes.