Sellers -- whose father was diagnosed with stage four pancreatic cancer just two weeks after Christmas in 2012 -- said this birthday was tougher to deal with on an emotional level as she knew it would be the last time she would receive a present from her dad.
"Ever since my dad passed away I've hated my birthday, and I dread it every year, because he always made it so special," she told the New York Post. "I was especially dreading my 21st, because I knew it would be the last year."
In the letter, Sellers’ father urged his daughter to be strong in his absence: “This is my last love letter to you until we meet again,” he writes. “I do not want you to shed another tear for me my baby girl, for I am in a better place.”
“Be happy and live life to the fullest. I will still be with you through every milestone, just look around, and there I will be.”
Sellers' was emotional.
"I broke down. That letter was very, very awesome and sad all at the same time," she said.
Her father passed away six months after being diagnosed, and was survived by Bailey, her mother and three older siblings.
The Eastern Tennessee State University student from Knoxville posts pictures of her flowers on social media every year, but was slightly surprised that the tweet went viral this time.
The post has been retweeted over 337,000 times and received over a million likes.
“I honestly think that was my dad," she said. "He was trying to make my birthday spectacular."