I'm in the querying process for my completed novel, and I completely understand how you feel. So, let me give you the good but double-edged news:
Even the best writers deal with intense self-doubt. Even writers from Sylvia Plath to Neil Gaiman have written about how that fear never really goes away.
I suppose you can see how that news can be both a blessing and a burden.
Insofar as writing a novel goes, it's like climbing Everest. How can you possibly do that? The answer is in the preparation. Before you dive into page one of your novel, you've got several incredibly important steps that are just as much a part of the writing process as completing your first draft.
Step 1: READ. Read what agents call "comps," comparable titles. Find other books like the one you want to write and read them. Not just the masters, mind you. Try to read up-and-coming authors as well. Read actively. Take notes. What do the writers excel at? Where do you think there should be more? Or less? Don't stress about whether or not you'll end up "copying" another person's work. Everyone has their inspirational authors, and authors are notorious for, not plagiarizing; let's call it learning about how you should write by reading what appeals to you.
Also, it's helpful to read a few craft books. My favorite, by far, is The Writer's Life by Stephen King. I'm actually not a huge fan of his fiction, but this book helped me substantially.
Step 2: OUTLINE. Speaking of Stephen King, he's the only author I know of (the only good one, at least) that simply sits down and starts writing. The vast majority of writers start with a bare bones outline. Who are the major players? What motivates them? What events will happen and in what order?
After the skeleton is put together, try writing just a paragraph or two of flat exposition about each chapter or group of chapters. Describe the scene and actions. Write down your own expectations for how each chapter will move the plot forward?
Step 3: Do your research! Make sure you know what you're talking about. Learn about the locales and time period of your story. Research the details of the jobs of the main characters. Not just what real people in those jobs do, but how they do it. You might know the gist of what a private detective does, but do you know the minutiae? The little things they've learned along the way to save time or to get better results? What sorts of slang terms do they use?
Step 4: WRITE. The most important part of writing a first draft is writing! Duh. What I mean is to not agonize over every sentence. I tell all my students, "All first drafts suck!" Even if you're the best writer in the world, your first pass on a novel will be awful in comparison to what you're capable of.
Just keep writing. Don't look back. You can fix the bad stuff later. If you're feeling writer's block, write anything. Literally anything. Just the mechanical act of typing can prime your brain to start getting creative. Body builders don't start on the highest weight; they warm up. Brains are the same way. I have a friend who has made his living writing poetry (poetry!), and he's succeeded by making a routine and sticking to it. He goes as far as writing at the same time every day, in the same notebook, with the same pen. It works! Eventually, just like working out specific muscles, your brain remembers the routine and will learn to prepare itself for writing before you even sit down.
The most important thing I can say though is to just understand that every writer struggles with self-doubt; the most successful writers aren't necessarily the ones that are the most creative, but they're always the ones that are the most persistent and committed.
I can say so much more about it, and I would be happy to help if you wanted to hire out my services as an editor.
Don't give up! Best of luck!