Amnesia, Golden Time Recommendations
Did you forget something important that you can't remember? Well pretty much I do, just kidding! Golden Time and Amnesia share the concept of the main protagonist forgetting everything about their past and have lovers that they don't know about anymore. If you want to find out more check them out. Note that, Amnesia is more a harem and Golden time has more love polygons.
Both have "amnesiac" characters and leave you with the question of a "what-if" love interest. There are tons of flashbacks and the main character in each story experiences extreme nostalgia.