Picture this. Your oldest friend dies. You dragon child(Viserion) dies. Child 2 - Rheagal, dies. Your fleet destroyed. Your best friend - murdered. Your boyfriend - well he has a better claim to the throne, and people like him more. Even though he says he doesn't want the throne, it feels like things are moving towards that being a possible outcome.
All of these things would absolutely wreck a person emotionally. Further, every time you try and do something the way others want - it ends disastrously for you. ENOUGH.
Dany is alone, and tired. And I'll say this - Emilia's acting this episode was, to me, fantastic. I could feel the desperation with Jon, the sudden hardness and ferocity when she told him what to do - the pain of Rheagal's death - and in the end, the fury of Missendei's death.