It's always a heartbreaking when a child dies before their parents, there's the sadness that a mother feels, the tragedy of a young life cut short too soon, and a heartache that nothing can fill. These are what Cindy Mathis from Georgia was left with when her 16-year-old daughter Macy died in a car accident. But after the tragedy, Mathis discovered that Macy had left her with something else.

Mathis remembers her daughter as a second mother, who would help her look after her three younger brothers.

Macy was also very creative, as you can tell from the last birthday gift she gave her mother: a set of letters.

Each letter had a message on the front: "open when...you miss me," or when "you're excited" or when "you're lonely." Mathis read a handful of them, then put the collection away.

Macy's car accident was just days later. After living in grief for a month, Mathis found one of her daughter's letters and started revisiting the collection, starting with "open when...you can't sleep."

"I want you to know that I love you," Macy had written, "I feel like I don't tell you that enough. And since you can't sleep and you're probably stressed about something, I need you to know that I love you"

Click the next page to find out what the other letters said!

Mathis was amazed by how thoughtful and true her daughter's letters were. "They're so on point, she told CBS News, "and there are occasions where she knew me way too well. My daughter knew me." Mathis still follows the instructions on each letter, and when she finally opens one she shares it on a special Facebook page. She says the letters help give her closure, and even seem to bring her daughter back for a little while.

"Each [letter] has helped me in so many ways," she said. "It's like she knew, but she didn't know... It's an amazing feeling. It feels like she's there with me."

In one letter, titled "open when...you miss me," Macy writes:

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

Mathis says that other teenagers have written to her, asking if they can borrow Macy's idea and leave letters for their own parents. She always says yes, because the letters have meant so much to her.

"It's the best gift that a child can give a parent."

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