1) That English writing major? Doesn't help one bit. Neither did that erudite senior seminar paper "Analyzing Literary Analytic Style: The Major Works of Stanley Fish's Literary Criticism." Means nothing in the real world of publishing. (That was a real title for my real senior seminar class that subsequently cemented my desire to study harder for the MCAT and go into medicine and NOT shoot for a masters in English.)
2) I thought I knew grammar, but apparently 4 years of college and I still don't know where to stick a comma. That's why God made editors.
3) Just when you think the book is perfectly logical and well-thought-out, some smarty pants comes along and totally punches holes all through the book. Stuff you hadn't even considered but then later are like, "Yeah, why DID the hero suddenly leave? That makes no sense."
4) A good editor is worth their weight in gold. And chocolate.
5) It's easier to negotiate a book series when you've already written the entire series.
6) It's fabulous to have great ideas. It's another thing entirely to get them written down. Coherently.
7) Not a clue about marketing for a romance novel. Not one clue. Still just clicking "favorite" and hoping for the best. Reading lots of articles helps, too.
8) Just when you think there's no way they can fit more muscles on that dude on the front cover, blammo, more muscles. And it's at that point you realize : the cover is better when it's super sexy, not when it's super accurate. And that's a-ok.
9) Don't argue with the publishing machine. They've done this a million times and obviously it works or they wouldn't be in business. You have done nothing prior except for a publish a poem in the Yalobusha Review. (Yes, that' s a thing and yes, I was in it, once upon a time.) Until you become a Nora Roberts, it's ok to defer to someone with more knowledge. Within reason, of course.
10) There's tons of free advice, and most is worth exactly what you paid for it. Pick and choose mentors and information sources carefully.
Bonus: Writing the book and finishing the book is only the first 10% of the job. The other 90% is selling the book and writing more books. (And eating chocolate, but that goes without saying.)