Someone is telling themselves a painful truth here: the person they love doesn't care about them, "not at all." The speaker seems to be giving a lesson to themselves. The opening line, "it's a hard concept to grasp," sounds less like a statement of fact and more like an admission that they still can't quite accept it.
The poem is built on a tug-of-war. The speaker knows they "must let go" of everything they're holding onto, but the last two lines pull the other way. After all the hard logic, what they want is something small and tender: to lie in this person's bed and "figure out life" together. That wish isn't dramatic or grand, which makes it hurt more.
The mood is quiet, raw, and a little resigned. Short lines and plain words make it feel like a thought spoken out loud, with the speaker still caught between what they know and what they feel.