Dear Little Me,
I know you think you know everything.
You don't, and that's okay.
Depending on when you see this, you are either a greedy child, or you act like wanting something will kill you.
Stop.
Opinions and desires are things that you will have. Let them exist. But don't waste your demands on petty things like clothes.
I will tell you this now: You do not have yourself figured out. There is a whole new dimension of you that you do not know of.
You do not love yourself enough. Stop doing that.  
Stop thinking that you are always the good guy. You aren't. You will do things wrong. You will be unkind. Accept that you did something wrong, and apologize if you hurt someone.
Most of your problems are not that big. Things can and will get much, much worse. But they will get better.
Stop worrying about other people's opinions. Embarrassment is fine.
Love, 
 Your Future Self
P.S. Please write back. I would enjoy laughing at how ridiculous your priorities are.

Dear Big Me,
How is life?
Where do you live?
What are you studying in college?
What is your job?
What are your hopes and dreams? 
I hope things are going well. I hope you are satisfied. I hope you got what you needed.
Can you tell me what you think of me? Please don't be too harsh. I know I'm not perfect. I know this letter was a random string of questions and hopes. 
Please make a good life. You deserve it.
Love,
Your Past Self
P.S. Please write back! I would love to know how your life turned out.