Make sure he's worth it; that guy you'll lose sleep over. The one you'll choose to have your heart and do with it whatever he pleases until you realize that you need it back. Make sure he's worth the weight you'll gain when you start skipping the gym just to spend time with him, and the weight you'll gain when you start eating heart-shaped chocolates by the box while you cuddle a pillow alone, watching romantic comedies where everything goes wrong but, somehow, still ends up right.
Make sure you understand that when he doesn't chase you in the rain, or down his driveway, or text you back, or ask for a second chance, that's the same as everything turned out right in those movies you love. This is just the kind of right that doesn't make money in the box office but does make a heck of wonder out of you.
Make sure he's really worth it.
You'll start praying for him more than you do your own family, shamefully so but incredibly true, so make sure he's worth all the blessings he'll never have any idea are coming from your forehead pressed to the floor in sujood. Be prepared to think about him a lot. Like, a lot, a lot. Like so much you'll worry you're not really over him, three years after you've already convinced yourself four different times that you are totally over him.
You'll think about him when you're lonely, which will be often. You'll think about him when you see a full moon and remember his eyes or the way you both looked at the same moon from different parts of the country that one time you romanticized how lucky you were to be a girl in love in Hawaii, while he was still on the Florida beach where you fell for him.
You'll think about him a lot while you're PMS'ing.
You'll cuss him out at random in your car, and cry at small things he used to do like looking right past you, and never holding you tight enough.
Make sure he's worth it all. Every tear, every poem, every diary entry, every whisper to the stars, every song that makes you think of him, and every dream you wake up from and text him after, "Hey. Just dreamt about you again. I hope you're doing well. Miss you." Make sure he's worth that "Miss you" because you will miss him, no matter how badly you beg yourself not to. You'll love him long, long after you are not in love with him anymore, so make sure he is worth being the one who teaches you the difference between need and desire, heartbreak and healing pains, loss and overcoming.
Make sure when he calls you beautiful, and you believe him, you thank him somehow for being the first person to ever make you believe in that fact. Make sure you are honest with him for why you leave, or you'll never forgive yourself for it later. Tell him "it's not you, it's me" because you will finally understand what the hell that statement means, and why it's so popular. You will most definitely be the problem. He will be busy living the life of the carefree twenty-something bachelor, gorgeous and talented, on a path to everything he deserves and more, and you will be there right behind him with your prayers and your love and applause. But no matter how badly your mom and your little sister want him to be the one because he will win them over with his charisma and his impressive sneaker collection, you will not be on that path beside him. You will realize that although a year older than you, the boy you are in love with is not the man you have been waiting for, and you will know it before you'll know how to tell him.
But make sure you tell him anyway. Practice saying the phrase "I love you, but" in the mirror until your jaw aches and you can say it in your sleep. Practice what comes after the "but". Practice telling him you deserve more than love that isn't coming back to you. Practice telling him that nineteen years of self-love has taught you exactly how to be enough for yourself, and you are not willing to compromise an ounce of your worth just to justify his. Tell him you can't, because as much as you'd like to pretend differently, that self-love did not come easily. It's been a marathon of eating disorders and body dysmorphia fits of rage and family histories of mental illness, name calling, and backbiting, and crying and crying and crying. None of which he will know about because, well, he never asked.
And that is okay. It is more than okay because it will make leaving so much easier. You will not be leaving any of yourself behind that didn't already belong to him before you knew him, and while it won't help the pain, it might help the act of healing from it quite a bit. So take full responsibility for wanting more from your life than he can provide, and let the rose colored glass of the house you built for him shatter as you slam the door on your way out of it.
This relationship, your dear heart, is a broken thing to be proud of.