First of all, believe you can write. Don’t let anyone’s opinion convince you otherwise. Poor contest scores or bad critiques can make you think about ending your writing career before it even gets started. My worst critique was done by a woman who said I had so many errors that she didn’t have the time or energy to critique more than a chapter, let alone a complete manuscript. Thankfully, I thought this was a rather outlandish thing to say to someone, sloughed it off and kept trying. It is a well-known fact that if you continue to write you get better. My writing improved and my manuscript went on to sell. Not everyone is going to like your writing, it is really all subjective; people don’t all like the same type of story.
What should I write about? Don’t worry about writing the type of story that is selling right now as something else may be in vogue when you finish. Instead, write the kind of book you love to read and add what you know or are interested in learning. Further, stretch your horizons. I thought I could only write fiction, but then found I could also write an article on something I knew about, and sold to a magazine, twice, before I sold a book.