Just finished Witcher 3 and got the bad ending. Properly traumatized.
Edit: As far as I can understand, the 3 places I went wrong were: Telling Ciri she doesn’t have to be good at everything, joining as backup with the mage meeting, and not trashing the lab (I thought the necklace scene was sweet).
My first play through I got the same ending, but I didn’t take as bad ending. I thought it was a cliffhanger for a DLC or another game.