Treat this like a passion & job both. When you are WRITING, it's time to let your passion & creativity flow. When the book is done, you need to switch to business mode.
Letters to agents or editors should read like professional business letters. Discussions of cover, release, website, etc etc, all have to keep in mind that this is--like any career--subject to the whims of the market. Don't let your creative self get in the way of professional interactions. Sometimes that means writing replies but SAVING them as drafts for 48 hours so your mood can level. Being a professional writer (or pro artist, singer, etc) involves switching modes between creative and practical.