I read on Wiki that the song was co-written with Adele about her life from her perspective.
When I read the lyrics it seems obvious that it has something to do with the pain she suffered at the hands of some kind of abuse. Whether it's mental, physical and or sexual is not clear. I don't think it matters. The point of the song, is quite clear...despite all of the pain and abuse, she's alive and breathing. I take that to mean more than mere survival, but something along the lines of her vindication as a human being.
"You took it all, but I'm still breathing..." meaning, you took everything you possibly could from me, mentally, physically, emotionally, sexually...and despite that I have overcome your abuse. I'm Alive!