I would say "君のこと、きっと忘れない" Kimi no koto, kitto wasurenai. Japanese is not my native though, just an opinion :)

あなたのことを わすれません Anata no koto o wasure masen. ...There are many way to say this in Japanese. ①あなたのことは わすれない ②きみのことを わすれない ③おまえのことを わすれない ① is used when there's no reason to be polite between the speaker and the listener. ② is used when the speaker is senior to the listener. ③ is used when the speaker is male and senior to the listener. ...Each other way has danger to be impolite or discriminative. So I recommend you the most polite and gender-free way "あなたのことを わすれません"