To my past self,
I know we've spent a while together, years in fact, but I just want you to know I don't want to forget what's happened in my past, but I also need to look forward to the future, because soon my future will become my past and I don't want to look back and wonder what I have done with my life.
I'm not proud of everything that I've done, I wouldn't be human if I was, but I would like to think that maybe in my future I can look back and be proud of most things and how my life turned out.
From my present, your future, self.

To my present self,
My first piece of advice to you, even though you are me,is not to dwell on the past too much, because the past is just that, the past, but it's ok to occasionally, otherwise you'll lose sight of who you are.
My other piece of advice for you is don't always live for the future, live for the present too sometimes because you don't know how much time you'll have, but also plan for the future so that you know where you are going in life and you'll know just how to get there.
I know we are always together.
From my, your, present self.

To my future self,
What have you done between when you are reading this and now when I'm writing this? Then know that some of the decisions I have made in my past and in your past have been difficult, as you know, but I hope they've been the right ones as I hope the life you're living is the one that I want and is the right choice.
I hope that you have had the courage to be yourself because I know I haven't always. Remember what I have done, remember what I am yet to do. And above all remember to always be yourself, even when it's hard.
I hope you are proud of me and what I have accomplished.
From my present, your past self.