Both include love that transcends multiple lifetimes while some characters also retain all their memories.

This is a movie as opposed to the drama, but this also shares a similar point of love transcending different people during the same lifetime.

Both have xianxia and wuxia elements along with the idea of falling in love between different lifetimes with the same couple.

Two boys and one girl from the same class. One boy goes through something traumatic leading to the revenge plot. Both boys also like the same girl.

Both dramas have an older male lead that is being chased by the female lead first. Both female leads also have terminal illnesses.

Both flashback between the present day and their high school days eventually closing the gap between the two periods. The main leads in both constantly reminisce about the past. If you don't mind crying as I did for First Love, 20th Century Girl is a good watch but be prepared with extra tissues!