I would tell my younger self to make sure I research every aspect of writing before stepping into the field. I think one of the big mistakes a novice writer makes, as I myself did, is focus only on the writing but you need to know about submissions, cover art, marketing. I think that can hinder one's success if you don't have knowledge and a plan of action.
I would want it to be any of Jane Austen's earlier works. I found her to be talented and clever. I feel her work is often underestimated but she was brilliant and so with her time period.
If I had to change one thing about myself I would have worked on my shyness a lot sooner in life. Being shy has always made it difficult for me to reach out to people. I think that's why I try so hard to make them feel connected in my books.
I think the greatest success a writer can have is to hear that a reader has loved the book. If I can bring someone on this wonderful, sometimes emotional journey than I feel like I have done my job.