Responses
No, sorry. The story was shorter than that. Besides they are already in a relationship, there's no expectation nor developing it there. Also, the guy is already a grown up teenager... The story I'm looking for started on the cute side because like I said, it was about a little kid confesing to his crush.
Manga where there was this little kid (or young boy?) in love with an older girl, but he was determined and swore to marry her and make her fall in love with him when they grow up.
I don't remember if he succeeded or not.