Celine Dion got emotional when candidly speaking about being diagnosed with stiff person syndrome. The 56-year-old My Heart Will Go On singer addressed her career-ending neurological disorder in her upcoming documentary I Am: Celine Dion, revealing what she misses most is performing for her fans, RadarOnline.com has learned. Dion’s eyes welled up with tears in a powerful part of the trailer, released on Thursday, as she discussed coping with the harsh reality of life off the stage. “I am working hard every day but I have to admit, it’s been a struggle. I miss it so much. The people. I miss the…