Hell, Mikasa learns, is a survivable place that she often takes up residence in. Hell teaches her she can watch her comrades be eaten and not turn back to save them. It proves to her that her she can be hungry but still functioning at a hundred and ten percent. Hell lets her know that she's human and forced into a situation she never particularly asked for. She learns the unfortunate truth that she doesn't have to die to visit the Inferno.
No, Hell has done a spectacular job of following her quite well in her mortal life. To her dismay, Hell teaches her some of the hardest and most trying lessons a mortal can learn. She will die as everyone she loves has; there is no truth more honest than this, she knows. And, while she's learned this at a young age, she'd thought—at least—she could protect her new family better than her last. Hell is learning she can't keep her adoptive family any safer than her biological. But the last time the Inferno greeted her it was different, for she hadn't been the lone survivor—Eren had made it out with her, bruised and devastated, but he'd been alive.
With Eren and Armin in the aftermath of losing her second family, she learns that Heaven exists with them as well. It comes in smaller packages and is often less noticeable, but it's there. Sometimes it shows up in the form of a loaf of bread or a bed with a wool blanket. How ironic, she thinks the first evening in the corps while looking upon a warm bowl of soup and fresh bread before her. How could she have known she'd find Heaven in a place about to send her to Hell?
Heaven is also a roof over her head when it's raining. It's the security that she has a spot to call her own again. It's the comfort of knowing Eren is, for the moment, safe while training—or so she thinks, anyway. Hell likes to remind her once in a while it still exists, too.
Hell reminds her not to be too complacent or to let her guard down for too long. It reminds her of its presence by wiping Eren from her life in a single, giant mouthful. Armin's tears look to her like small drops of hellfire and it's all she can do to save him from the same fate that Eren met, to keep him safe from the monster that has followed her for years. And as she leans down to talk to Armin, to remind him to keep going, she's thinking all the while of the monster that has its hold on her life.
But Heaven is sweeter and more apathetic than Hell; it also, in its subtle ways, reminds her that it is around, too. The definition of Heaven to her is the sound of Eren's heartbeat, loud, clear, and so very alive. Heaven is the relief of holding him in her arms and crying so loud she thinks she might burst; but surely that would never happen, for the stars are finally in her favor.
She would be lying if she said she wasn't the least bit scared of all the guns pointed at them, ready to fire at will with a single command. But she isn't so scared she won't stand up against them, because while Hell is at her feet, Heaven has her back; the monster that is the Inferno will not consume her.