Think about it, you are halfway through your first draft and you keep staring at you work saying, "If only I had planned more, if only I had thought more about the climax." If only, if only, if only...
You regret the way wrote this scene, you start to doubt if your story is even very good. Then, you set your story aside for a little while and think it over, but you only think about more downsides. You start to get bored of writing your book and lose interest in it. You start to think about throwing it away, you start to consider giving up.
But, keep calm...it's only the first draft.
The truth is, this was your first time writing this plot, this idea. It is like when you play a game for the first time. Can you guarantee that you play every move perfectly? Can you guarantee that you will win? It is just like writing a story. When you are fairly new to a plot (although you may have done lots of planning), you can't write everything perfect; you cannot win in your first draft.
That is why rewriting and editing was created. Although you might cringe every time you look back at your book, just keep writing. You can clean up your work after you get your messiness blotted out on paper.
Finish that first draft. No matter how hard it is, how bad it sounds, you can always clean it up. You are not alone in the fight to finish your first draft. Keep calm and just keep writing. One. Word. At. A. Time.