I feel the same! I thought it would be fine and I wouldn't get emotional over it, and really it wasn't even the realization about anyone else that did it, it was Aang.
I got so excited over Korra's character because I just loved her personality, and I still do, but then about halfway through the episode I realized that her very existence meant that he had died. It hurt my heart, especially because in my mind he's permanently a twelve-year-old boy.
It also made me feel for Tenzin, now he's the only air bending master in the world and he has a responsibility to carry on that legacy... ;3; I'm sure that weighs heavily on him.