That.. Is really formal. 당신 means "You" but is obscure and archaic. "사랑하는" isn't formal or informal, but means that you LOVELOVE someone, which maks you seem sorta.. you know, since you r a girl and we r mostly girls (mOSTLY). That dos not say saranghae, but "dangsineul saranhaneun" which is probably grammatically incorrect. I would recomend:
-사랑햬 (saranghae, casual and friendly)
-사랑햬요(saranghaeyo, casual and polite)
-사랑하다 (saranghada, neutral, justmeans "to love")
-너을 사랑해 (neoeul saranghae, "to love YOU,"
But I AM just a student, so I could be wrong. (PLEASE DON'T USE THE LAST ONE I JUST CHECKED WITH A REAL KOREAN) I admire your love for th culture and language, though! Koreaboos 4eva!