First off, I totally get where you're coming from. Doubting your work, especially when you're putting in so much time and effort, is totally normal. I've been there too. So, here's the thing - feeling like your book is bad or your idea is weak is part of the process, especially when you're pushing yourself to improve.
Here's a tip from my own experience: It's crucial to have a clear plan. If you're changing things as you go, it can lead to confusion and doubt. Stick to your plan, and if you do want to make changes, jot them down for later. Don't try to edit while you write – it can turn into a big mess.
Finishing the draft should be your first goal. After that, you can dive into the editing process. Two ways to do it: jot down notes as you go and edit later, or stop writing, edit your plan, and then continue. Trust me, trying to write and edit simultaneously can get messy.
Once you finish your first edit, take a breather. Think about the whole book, consider any deeper views you might have missed. I usually take a few weeks to reflect. Then, dive into the second round of editing. Take it step by step.
Remember, I've found that the stories I was unsure about were often the ones readers loved the most. So, don't be too hard on yourself. Your job is to do your best and finish what you started. Let the readers be the judges. Keep going, and good luck!