Romance, Dual Timeline, Backstory features grandparent of one of the MCs who is from a fairly wealthy family.

[Keiichi]

Great day to all of you, I'm trying to remember the title of a book I've read around 2018 or 2019. I think it has a dual time line which involves the grandmother of one of the MCs. In grandma's time she is the daughter of a fairly wealthy family, her parents didn't want her to continue studying and instead marry someone they approve of, she met this guy who drives a motorcycle but i think they ended up parting ways due to the guy enlisting in the military. I think grandma either tells the story to her grandchild or has left a letter telling her story. I'm hazy about the other details but hoping that anyone can help me out. Thanks.