Erwin was all you had, your only family, the only shred of love you had left in this terrible world full of titans. You were his world, all he had, and the only thing that kept him sane. Unfortunately, you knew that his role as commander in the Survey Corps meant that on any given day he might not be coming home.
It hurts you to know that maybe one day, while you were baking a cake and waiting for him, he could be dead. Erwin reminded you of this several times before you started a relationship. You were perhaps too stubborn, or just crazy to accept this. But what to do? Erwin loves what he does even though it is extremely dangerous.
Now here you were, waiting for any sign that your partner, your lover, was alive. You were in your shared home, Erwin's traces still all over the place. You held the letter he left you tightly to your chest while waiting for him. Erwin had left home on a mission, a dangerous one and he was definitely taking longer than usual to come back. Until finally you heard a knock on the door, and suddenly you felt hesitant to open it.
It could be your lover, or it could be a soldier giving you the worst news of your life.