No matter how much Bernie drinks her mind is still trapped, trapped on that night. She's standing outside that door, banging over and over calling Serena names. When she tries to trick her mind into giving that night a happy ending it doesn't work.
Serena slipped. It was the wine. No matter what the others say. She slipped from that roof, she didn't jump. Bernie tells herself that over and over until she believes it. Until it becomes the truth. Serena loved her too much to jump and leave her, slipping was the only thing that made sense to her.