"Open this when you miss me." The letters a grieving mum found from her dead daughter.

It was 10:30 at night, only weeks before Christmas. It would have been cold on the roads. Dark. The car was travelling North carrying two passengers – Macy Mathis, 16 and Adam Sattler, 18. Boyfriend and girlfriend, both from Tifton in the US state of Georgia.

They were travelling through Tifton when the car struck the side of the road. The driver, Adam, lost control and the Nissan wrapped around a tree.

Adam died at the scene. Macy was airlifted to hospital where she was in a critical condition for five days before she, too, passed away.

Christmas didn’t feel like Christmas for the family and friends of the young couple. Moments of laughter would have been shadowed by suddenly remembering, realising those two who were so cruelly, horribly absent.

But, last week, Macy's mum Cindy fell upon a stack of letters she forgot she had from her late daughter.

The letters were a gift from Macy, to Cindy for her birthday, just a week before the accident. It is understood Macy was not living with her mother at the time.

They were written with instructions, such as "Open me when you miss me" or "Open me when you can't sleep".

Cindy has opened three of the 25 letters, and shared an image of one to Facebook.

"This one was so hard to open," Cindy posted. "You don't know how much mommy misses you! There is not a second of the day that goes by and I don't think of you."

"You are my rock, my best friend, my everything. Your brothers are doing such a great job holding me, Hope, and your Daddy together. I am very thankful for the family YOU created before and even now. We love you so very much baby girl!! Mommy misses you more than words could ever express! Goodnight my sweet, beautiful angel."

The letter from Macy gives you goosebumps. Now, more than ever, the message is so, so poignant.

"I'm sorry that you're missing me," the letter reads. "I hope wherever you are or whatever you're doing, you're okay. I'm probably missing you too."

"I will always and forever love you mummy. Thank you for everything you've done for me."

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As soon as she remembered them, Cindy "tore apart her room" looking for the letters. Now, she says the messages are helpful in her grief.

“Most of them are definitely definitely uplifting,” Mathis told CBS News. “They’re so on point, and there are occasions where she knew me way too well. My daughter knew me.”