Okay, this quote is probably, maybe not totally all about nobility. It's so much deeper than that. Truly understanding this quote, or truly living by it, involves a lot of self-reflection. But when you really think about that task and think about true self-reflection, it's a much more difficult task.
Looking inside of yourself and addressing your past mistakes takes a lot of work. Some can do that easily, while others find it hard to reconcile who they were with who they want to be with.
Hemingway's words above are a great reminder to always reflect on who you used to be, on who you are now, and who you might become in the future.