As a graduate student, I wish I knew that respected scholars go back to the drawing board to learn new things — new methodology, new theory, new content – -throughout their careers.
I wish I knew what everyone told me I would come to see (but it really took time): that rejection (of manuscripts, book proposals, conference proposals, funding proposals) is a really normal part of the job for everyone from students to heroic senior faculty. The first rejections I encountered felt like huge setbacks, while now they feel like developmentally appropriate, often very instructive stops along the pathway (I also wish that all reviewers were like the reviewers who give really specific feedback, even if it is hard to take in). Many people who end up in academic positions get there because they had relatively few crushing rejections as doctoral students, and got articles, conference proposals and funding proposals accepted. Once submission of intellectual work is a central part of one’s job, however, the opportunities for rejection only increase!
My advice is more like successful strategies for grad students, rather than what I wish I had known: Find great collaborators who have the same project timelines and working styles as you do, and work closely with them throughout your graduate career.