"
it's like you're my mirror
My mirror staring back at me, staring back at me
Aren't
This part of the song, "Cause it's like you're my mirror, my mirror staring back at me," means that Justin sees something in his lover that he also sees in himself. He believes that they are so perfect together that if they were to look into a mirror that showed the quality of ones soul, they would be the same because that's how perfect of a fit they are together. He believes that if he were to look into this metaphorical mirror of souls, he wouldn't be able to tell the difference between himself and his lover.