http://www.mangago.me/read-manga/ore_no_kanbotsu_chikubi_ga_senpai_no_shitapi_de_tachimashita/ the not announcing relationship happens a bit later
feelings about ex:
http://www.mangago.me/read-manga/fools/
http://www.mangago.me/read-manga/kakkou_no_yume/ this one is sort of in that vein but its kinda complicated lol
http://www.mangago.me/read-manga/inner_beauty/ this one the guy is dating his ex girlfriend but they eventually start liking each other
i hope these help a little?
Yaoi manga where one of them gets upset because the other doesn’t want to announce their relationship?
Or the other still has lingering feelings for his ex and it causes a fight. Stuff like that