Isekai Shoukan Sarete Kita Seijo-sama ga "Kareshi ga Shinda" to Naku Bakari de Hataraite Kuremasen. Tokoro de Sono Shinda Kareshi, Zense no Ore desu ne.

Reincarnated as the third prince of the peaceful island nation of Astraea, Edward has lived a free and easy life. However, his life changes drastically with the appearance of a saint summoned by the goddess. Edward is chosen to be the caretaker of the saint, instead of his more handsome and accomplished older brothers. It is a great responsibility and quite troublesome to take care of the saint, whose mental state is manifested in the weather, who keeps ignoring the missions given by the goddess, while crying and making it rain all the time.

When Edward asked her why she keeps crying, she replied, "My boyfriend died."
Edward knows, because he was that dead boyfriend in his previous life!

A reincarnated prince and a saintess get a second chance, in this isekai reincarnation love comedy!

(Source: Tugikuru Books, translated)