I think authors get hung up on their marketing because they don't devote as much time to learn about the industry and their genre as they do on the craft of writing. For some reason, marketing is relegated to the back burner in such a way that when the book is done, there's a "sitting idle" time.
When I speak at conferences, I often remind authors that when they transition from being writers to authors, they transition from artists to business people. A writer, writes. An author sells her writing and a successful author does both well.
The book is paramount to the success, so make sure it's edited, polished, set aside for a week, then edited again after several beta readers, and critique partners have made it bleed. But once the book is in submission, your marketing platform needs to be strong in order to help your agent, editor sell the book to the publishers they are courting with your manuscript.
The old adage, "It's not what you know but who you know" applies greatly to publishing when authors are looking to market their books (regardless of the publishing route explored). When you're not writing, you need to be cultivating relationships with readers, reviewers, book industry people online and in person. This is where conferences and blog tours come in handy.
Research blog tour companies and talk with published authors in your genre who have recently used a blog tour company to get an idea of their experience and learn what works and what doesn't. If you're represented, reach out to your agent's other clients who write in your genre and introduce yourself. Pick up their books and familiarize yourself with their writing style. Is it similar to yours? Do you think their readers could be part of your target audience? If so, reach out and ask for information. Most authors like to talk about their experiences in the publishing industry (some a little more than others) and many like to help new authors or mentor them in some way. But be authentic in your desire for mentorship and/or help. If they are unwilling or unable to offer more than a recommendation for a blog tour company, for example, say thank you and engage with them via social media about anything BUT your book.
Expect review sites to take MONTHS to get to your book, so make sure to send out ARCs early and to a large number of sites. Cast a wide net. Join Goodreads groups that allow you to connect with readers. Talk to your publisher about reducing the price of your first book so the second gets a chance to pick up new readers. KNOW your genre and the heavy hitters in the genre and get to know what works for them and doesn't. When you're not writing, you need to be promoting. Set time aside each day to do something to promote yourself. Readers want more of you, they buy your books to get that so give them YOU!
Then start thinking outside the box for your marketing. Look for opportunities to talk about your book on social media without panhandling it. Talk about your writing process, your cat, your cat's inevitable desire to thwart your writing process at every turn. Anything that's not just blasting social media with your book every five seconds.
Then sign up to speak at conferences and partner up with other authors in your genre to help expand your reach. Be active in promoting yourself as an expert in your craft because you are! You have expertly written a book, sold it to a publisher and expected to sell it to thousands of readers. What works and doesn't work become part of your expertise. Talk about that at conferences and take these opportunities to also engage your readers one-on-one. Sign up for books signings at conventions. Schedule speaking engagements at our local writing group meetings. Get out there and make your name known!