unfortunately no word(or phrase) is same meaning with "I'll miss you" in Japanese... cuz "Ill miss you " means "I'll feel lonely cuz u are not here" or "I wanna see you" or "I wanna feel you " or ,,,like that right?? ( Im sorry , firstly Im not good at English) I miss you nearly equal 我想你。 but we Japanese dont have that kind of expression. so we choose those expression depending on situation. "I'll feel lonely cuz u are not here" anata ga inakute sabisii desu. あながいなくてさびしいです。 "I wanna see you" anata ni aitai desu. あなたにあいたいです。 etc.....

I'll miss you. （あなたがいなくなると）さびしくなります。 The person is just about to leave, but not gone yet. I miss you. （あなたがいなくて）さびしいです。 The person is already gone.

あなたがいなくてさびしいです。

kimi iku to sabishii yo.