Let's get naked here, shall we? I will bare my soul (and my bum) and tell you about the horrible marketing sins I committed, and you will gloat and snicker and hopefully learn on my mistakes ("Don't eat that apple, Ksenia!" you'll scream. And I'll say, "TOO LATE," eat half and force the other half down your throat.)
So. Marketing. Marketing is like breathing when you self-publish. Without marketing you won't sell, and without selling you won't thrive. And without thriving...well, you'll be tempted to quit. Too many self-published authors do. I almost quit the whole affair myself, and not once, three times (from the times I remember). Whatever ignorance or arrogance or both compelled me to think that now that I was writing books (and not running my start-up), I didn't have to market? I thought my books would market themselves. I thought my stories would be so good, they'd compel my readers to market them for me. And I was right and wrong at the same time. Right in that, YES, my readers marketed for me. Wrong in that, NO, they didn't do it on their own, so once I stopped asking and keeping in touch, they stopped doing it and forgot about me and my books.