Jesus, you people have no chill.
Like, there is a whole host of explanations for that last panel that aren't "she's sleeping around" or "she killed herself".
She probably, as mentioned, -went to bed-.
Yeah, this story is sad because the protagonist is hurting her girlfriend without realizing it, yes, it is partly the girlfriend's fault for not speaking up, No, there is no reason to immediately jump to the worst possible conclusion just because the girlfriend isn't in their living room for a panel.
This is a story about how working hard to support someone financially can lead to neglecting them emotionally.