Ah, that painful process, where the euphoria of completing that first draft is swiftly replaced with loathing, fear and disgust as you re-read your peerless manuscript and discover it ain't so peerless after all. That it's more full of holes than swiss cheese.
Stinkier than limburger cheese or rotting gorgonzola, and twice as ugly.
A word of advice: relax. Accept that this is the process. It's a long, slow spiral of many drafts before you get to the heart of the last draft.
To use another image, writing fiction is a labyrinthine process, full of dead ends, sudden turns, obstacles and wrong turnings.
You can, as I often do, waste energy berating yourself with gems like, "If I were a better writer I'd get it righter first time around!"
I don't know any writer who does get it right first time around.
It takes the maze-like twists and turns to discover and uncover the story you want to write. It is all part of the process, so relax and enjoy it. It's absolutely necessary to take those wrong turns in order to find the right ones.
Often, that first draft is just scaffolding. Necessary to tear down, but absolutely crucial to build the stunning structure you will end up with.