Dealing with stage 4 breast cancer is bad enough, but Heather McManamy from Wisconsin believes that leaving her four-year-old daughter Brianna motherless for the rest of her life is much worse.

It breaks her heart knowing that she will not be around to see Brianna fall in love, graduate from college, take her first job, get married, and have a baby. But Heather has accepted the fact that stage 4 breast cancer has no more cure since the disease has already spread to her bones and liver.

To make up for her absence in the days and years to come, Heather decided to write letters that her daughter can read in the future. Whether her daughter is having a bad day or celebrating her birthday, she hopes that her words would give joy and comfort to Brianna.

"Her first tooth that she loses, her first wedding anniversary," Heather told NBC News about her letters. "Oh, I have one for her first baby. The last one I did was for her 30th birthday."

Brianna is heading off to kindergarten this year, and Heather hopes to still be around to read the actual letter herself. It was very difficult for Heather to write those letters, because she really wants to stick around and take care of Brianna herself.

"You know, it was difficult once I was done, looking at this pile that's this physical representation of everything I'm going to miss," she said.

But it does not matter if Brianna decides not to read the letters in the end, Heather said. What's important is that her daughter knows how much she wanted to be there for her. "I want her to always know that I did everything I could possibly do to be here," she said.

Some of Heather's letters contain memories the two shared together. One letter reads, "Every minute with you, you made me happier than I ever thought I could be."