I believe everything was well explained in the documentary feature.
- "Quite often, the illness comes not from the symptoms, but from the stigma, isolation and mistreatment that comes about from the rest of society".
- "(Senua´s) psychosis (...) was exasperated by stigma and isolation at the hands of the clansmen and her father. The trauma of seeing her loved Dillion sacrificed tips her over the edge, making her remodel her reality around a concept that connects everything: the Darkness."
A better explanation is given by someone that suffers from psychosis and played the game:
http://www.kotaku.co.uk/2017/08/09/hellblade-psychosis-and-me IMHO, this is what's going on: Senua, a girl born in an era of complete ignorance, fear, superstition, etc. with a condition which her society finds to be a "curse", has been loved by two people in total in her entire life: her mother and Dillion. Her father not only secludes her, he does not like that Dillion and her see each other and denies her even that. So, she goes on an exile, becoming a "Geilt", with the hope of getting rid of the "curse" so she can be accepted by her father and society and not to "infect" her lover. When she comes back, she finds her village destroyed and her lover brutally murdered, which triggers a full-blown psychotic episode, something we are playing in the game.
As the Kotaku article above says: "instead of conjuring up complete nonsense, these hallucinations play on your own personal fears, creating images that feel entirely plausible within the warped reality victims find themselves trapped in". So, in the end, the Darkness is the sum of her father Zynbel, her society, the life Zynbel had imposed her, the fear of eternal reclusion and loneliness, the rage against the Northmen who destroyed the village and killed Dillion, and fear to hurt her only loved ones, all augmented by the tales Druth, another Geilt she met in her exile, told her.
So, the entire game was an internal fight to finally:
1.- Accept Dillion's death (and her mother's).
2.- Believe him about what she had was not a curse, but "an alternative way to see the world", so there's nothing wrong with her.
3.- Believe that she can live with her "curse".