A California teenager who once dated Celeste Rivas back in middle school is speaking out ... saying she had a troubled home life before running away -- and her mom treated her badly.

In an exclusive interview with TMZ ... 15-year-old Damien Hernandez and his mother, Jamie, told us about his relationship with Celeste at a Lake Elsinore middle school ... Celeste was in the 7th grade, and Damien was in 8th grade.

Damien says he met Celeste when she was looking for something serious, describing her as "kind." He says her troubled home life made her unhappy, noting that her mom did not treat her well.

Damien said she talked about running away from her Lake Elsinore home -- and even joked about it around school. Then one day -- she was gone. TMZ.com

What's more ... Damien says

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