Question: Should I stand or sit while others recite Mourner's Kaddish?
Answer: I was also raised in a Conservative synagogue in which I sat while others recited Mourner's Kaddish in memory of their close relatives and I only stood for Mourner's Kaddish when I personally was obligated to say Kaddish in memory of someone Jewish Law defined as my close relative. I can understand that after being told "If you rise to recite the prayer and you did not really have anyone to mourn, then you will soon enough" as a child, you would prefer to sit. So I understand the tension you feel when now the whole Reform congregation around you is rising and you don't want to rise with them.
My first suggestion is for you to speak with your Rabbi. The principle is that one should observe the local customs and "not separate from the community."
Secondly, it may be helpful to you to know that when my grandfather
died, my father asked if I could say Kaddish for him in the mornings (he would say Kaddish at the Mincha/Maariv service). I checked with a professor who was a specialist in Jewish law and customs. The professor explained to me that the "rules" surrounding Kaddish are entirely custom. Since my father - and certainly then my mother concurred as well - asked that I do so, there would be absolutely no problem. I could also have said Kaddish for him, it would appear, even without my father's permission.
Thirdly, when I began to lead services and the Mourner's Kaddish prayer, there was no objection from my parents. These are the same parents who expected that I would not say Kaddish during their lifetime or stay in for Yizkor. They accepted without question that it was a professional role. They have reached their current ages of the late 80's, so clearly my recitation of Kaddish starting 50 years ago as a student Rabbi had no superstitious effect upon their longevity, thank God.