I actually write notes to my future self all the time. Usually they're kind of goofy. Or reminders to do things that I know a phone alert won't be enough for. Sometimes they're inspiring words I know I'll need later.
Dear future Shamwow (if you still go by that goofy nickname):
Listen to this song. Trust me, you still love it. A lot of stuff about you is going to change probably, but Patti Smith is forever. So turn this on blast and just dance. Do it.
Right. That was awesome, wasn't it? Look. I just want you to know, you don't owe me anything. That's how it is. Your past self is gone, she's not coming back. Future you though, she's gonna need your (our) help. See, putting off stuff like paying bills and getting insurance is just going to make her life harder. Forgetting to floss is just going to make her dental bills higher. Not doing what you love now is just going to make it more difficult for her to start again.
I hope you know that. I think I know that now, but only time will tell, right?
Gosh. There's a lot of stuff I want for you, future self. But honestly? I don't know if that's gonna make you happy. Remember what we were like at fifteen? God, nothing made that girl happy. All she wanted to do was die, remember that? Current me would have been really pissed if I didn't get to exist just because that sad sack wanted to throw in the towel. So again, while I really hope you followed that weird dream of ours, and finished that fic you've been working on forever, and made out with someone hot once in a blue moon, if that's not what you want out of your life anymore, then hey, far be it from me to tell you want to do. Paint ducks and retire if that's what you want.
I do need you to be happy though. That one's non-negotiable. Remember that terrible job you had in your early twenties? The boss that screamed at you constantly, the other boss that made homophobic remarks every other week. The heatstroke. The stress. That time you got electrocuted... the list goes on. You don't deserve that, okay. You don't. Never, ever, ever accept less than you deserve. You've done enough of that for one lifetime.
Good job getting this far. It's an awfully big adventure.
I love you.