Self-doubt - It rears up and can take over. It's those thoughts that creep in and make you question what you're doing. Am I wasting my time on my writing? Will I ever get published? Will my books sell? What if I get bad reviews?
It's good to review and assess what you're doing periodically. That's how you improve. But those thoughts shouldn't consume you or stop you in your tracks.
You have to be persistent. If you give up, you will never be published. If something isn't working for you, switch it up and try something else.
You need to acknowledge that writing is work. It is very rare that someone has a perfect first (or fourth) draft. Going to critique group, polishing the manuscript, revising, editing, and proofreading take time. To do it right, you have to put in the time.
As a writer, you will be required to market yourself and your book. It takes money, time, and energy. To keep fans interested in your social media sites, you need to post good content regularly and interact with people. But you need to balance this with your writing time.
You are going to get rejections and negative comments. Deal with them in your own way and then move on. Exercise is always a good way to work out the anger. Don't get obsessed about your number of followers, your sales numbers, or the reviews. Check on them occasionally, but don't let them take over.
Your job is to write your next book. Don't be paralyzed by the worries and doubts. There are always going to be challenges. If writing is worth it to you, you've got to commit to it and learn how to control that little doubting voice in your head. Work to improve your craft and write your next piece.