Kaia Gerber Pens Tribute to Late Brother Presley, 'I Wish You Hadn't Gone First'

Kaia Gerber is opening up about the devastating loss of her brother Presley Gerber ... sharing an emotional tribute that'll break your heart.

The model posted a lengthy letter to her late brother on Instagram Sunday, along with childhood photos and video of the two together ... admitting she's struggling to find the words to describe her grief.

Kaia wrote ... "i am, quite literally, down on my f***ing knees."

She remembered Presley as her protector and best friend, saying being his sister was always her favorite thing about herself ... and recalling how he stayed kind even when faced with cruelty.

In one particularly gut-wrenching memory, Kaia recalled being too scared to get her ears pierced as a little girl, so Presley went first ... making it all the more heartbreaking when she wrote, "when i held the earring they took from your body, all i could think was: that belongs to you."

She added ... "you always went first ... and i really f***ing wish you hadn't gone first this time."

Kaia ended her tribute by calling Presley the one true love of her life, writing ... "i was born loving you. i will die loving you."

The emotional post comes just moments after their mother, Cindy Crawford, shared her own heartbreaking tribute to Presley ... remembering him as a beautiful soul and thanking everyone for the outpouring of love and support.

As TMZ previously reported ... Presley was found unresponsive at a Santa Monica rehab facility September 20, just hours after checking in. Police have been investigating his death as a suspected overdose, with no foul play suspected.