The song playing is Tuxedo Mirage from Sailor Moon.
When I see you with another girl, it seems so unfair. You know that I loved you once, but now you don't care. I feel like you used me just to have someone close to you. I guess I was right, because you've said we're through. Now you're with her. Again, love is in the air. Why did this happen to me? Life is so unfair.