Help

I’m looking for a manga but I can’t remember the whole story and I’m not sure if it was isekai
The ml is the son of friend of the fl father and they meet when the ml and his father were visiting and then they fall in love and got engaged then the ml became at the age that he should enter the academy and as i remember he mut live there and can’t leave(Not sure about that) so they didn’t meet for a long time (may be 7 years ) but they were exchanging messages and when the ml has leaved the academy he became kind of cold(on the out side) and i remember that he was the strongest magician so the princess showed an interest in him but it was for a short time then she became the fl friend. Then i remember that something like a demon king appeared so they searched for the hero and they found him then the princess, the hero and the ml(because he is the strongest magician) and i think there were someone else(not sure) have gone to kill him then the ml used a spell or something and killed the demon king but the ml also died. Then the fl got really depressed. Afterward i think the ml was a live but disappeared for some time. Then the ml and fl got married but didn’t announced it, only thier family’s know
I can’t remember the complement