Read Becoming a Writer, by Dorothea Brande. Then do what it says, including the tasks you think are impossible. You will particularly hate the advice to write first thing in the morning, but if you can manage it, it might well be the best thing you ever do for yourself. This book is about becoming a writer from the inside out. Many later advice manuals derive from it. You don't really need any others, though if you want to boost your confidence, "how to" books seldom do any harm. You can kick-start a whole book with some little writing exercise.