My first novel taught me I could write. Seriously, I didn't think I could do it. I was a science major in college and I had one English professor who kept hounding me long after I took her class. Every time we'd bump into each other, she'd say, "Still waiting for your first novel to come out." I'd just shake my head. Didn't she understand I was not an English major (which I for some reason thought was a pre-req to writing a novel). Then one day I did it, started a novel. Once I was done, I wouldn't let anyone see it, until finally, a year after I had completed it, I handed it over to her and she loved it. I was blown away. I could actually write a book people liked. Two English professors read it, one I had never met, and he too loved it. I learned a lot about mechanics from thier edits on my novel. My manuscript looked like an artery had been severed and red ink soaked the pages. Aside from all my mistakes, they still seemed to find it an enjoyable read.
Basically, I learned so much about my own writing, about mechanics, pacing, plot, self editing, and so much more. But the most important thing I learned was...I can write.