The plot about Amasawa being Kiyotaka's fiancee is very interesting, not gonna lie. You asked for criticism so I will give you my honest advice. I saw names which not in big letters. Names, places, etc must be in capital letters. I also noticed the word "I". Most of the stories I read have the same problem as others. "I" must be always in capital letter.
Example:
"Kiyotaka ayanokouji, i will never forgive you!" (This one is wrong)
"Kiyotaka Ayanokouji, I will never forgive you!" (This one is correct)
There's also some words that is misused, but I think its normal since you said you're a beginner. Just make it minimize as possible. Put also comma and periods at the right place since readers will probably confused. There are some readers who have a high reading comprehension so that will no problem to them, but how about to those who have low? Well, Im just an amateur writer so take this advice as a grain of salt. Overall, the plot is interesting, just write always and your skills will definitely improve.