Ron Goldman's Sister Pens Emotional Note On Ron's 51st Birthday

"Happy Birthday to my big brother" ... 

That's how Kim Goldman began her letter to her late bro, Ron Goldman, on Tuesday -- while reflecting on his life 25 years after his death. 

"You would have turned 51 today - just insane to think about all of the years lost, absolutely breaks my heart," Kim wrote in a note posted on social media. 

Of course, Ron was brutally murdered on June 12, 1994 ... when he was only 25 years old. 

At the time of his death, Ron was working as a waiter at the Mezzaluna Trattoria restaurant in Brentwood and had gone to Nicole Brown Simpson's condo to return a pair of sunglasses that had been left at the eatery earlier that day. 

While at Nicole's home, Ron and Nicole were ambushed by a knife-wielding scumbag who killed both of them. 

Kim Goldman says she's still trying to focus on the good times she shared with her big brother.

"The what-if's torture my spirit, so instead I will focus on your smile, your zest for life, your kindness, the memories of us growing up, our talks, our laughs, our tears."

"I will focus on the tremendous adoration and respect I have for you and I will hold on dearly to the memory of what your hugs felt like, cuz I sure could use one."

She ended her letter, "All the love in the world, xo Squirt."