Why is believing in yourself so hard?

Sooo... I've had a crazy week to say the least. I should be learning for a "statistics in life sciences" exam, but every time I try to sit down and really do that, I twitch and suddenly I'm writing. And I'm not even writing what I should, that being "Unwilling" and "Shapeshifter", oh no. I'm writing a whole new story, and right now I'm topping out at about 5,000 words a day. Those of you who 'know' me can attest that this isn't me, and that it's really creepy.
I've just broken the 30k-mark on a story that doesn't even have a weekly anniversary yet, and I'm having suspicions about this. I didn't dare hope or believe or do that usual "yeeeh, you can do it!"-tango that I normally turn to when something goes well for me, but I'm still positive that this will be the first story to be finished.
This will be a break-through, and I thought I should celebrate it with doing some promotion for myself. I stared at patreon for a while. Then I surfed through a number of romance labels who might or might not be interested in publishing my works. And the Amazon self-publishing sites. And all the while I had this song in my head, a song I've been hearing since I was a teenager. You might know it.

"Do you really think you can do that? You're not a real author, you know that, right? Don't make a fool of yourself by trying to play with the big boys. You can't just click that and type there and make money, honey, that's not how life works. You might think your work is good, but if you click that, you'll find out that it isn't. It's not even a question of 'if' you'd fail, you'd fail. Don't do that to yourself. Stay where you are. It's cosy there, isn't it? Why would you want to change that? What if you really are successful and people suddenly expect you to do things you aren't comfortable with? You can't keep up with others who self-promote, you're not that good. Not that convinced of your work. This will end in heart-break. Don't!"

This sucks. I know my mother heard that same song in her head, she told me so when I was eighteen and failing high school because fuck this shit. She told me how much she envies my father who just... does stuff, and then other stuff happens, and everything falls in line for him. She always says, if she could master that skill, that "just doing it and to hell with what other people think"-skill, she'd probably rule the world in a year.
I believe her.
Difference: I WANT to rule the world. I just have to convince myself that I actually COULD, if I WOULD finally click that damn button!

Well... I'll give it another try tomorrow.