How do you know if your idea for a book is a bad idea or not before you start writing? I don’t want to get 20,000 words in and realize there’s a major loophole in my plot or it’s just overall a terrible book.
Constantly work on the plot. While writing, you’re going to come across ideas and realizations that will make it better. Don’t let those go by for the sake of finishing; if you have to go back and edit them in, do it. It’ll help you out immensely.
Get feedback. You don’t have to show anybody your rough drafts, but it will help a lot to find that group of friends to ask about plot ideas or just brainstorm with. Sometimes it can be tricky to find the right group – not all of your friends will really know what kind of things you like to right – but it can help a lot.
Take breaks. When you feel like shit about your story, don’t force it. Take some downtime. Do some other things to help you relax. The ideas will come and help you fix it.
Ask questions. To avoid plotholes, ask questions (how did this happen? Is that explained in the book?) To avoid terribleness, ask yourself questions about what in your book excites you. Focus on that! If you really like it, chances are other people will too.