Celeste Rivas' Middle-School Boyfriend Says Her Mom Didn't Treat Her Well

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.

What's more ... Damien says he heard about Celeste's alleged relationship with singer D4vd ... but was surprised because he thought she was happy -- outside of her home life.

As you know, Celeste's dismembered and decomposed body was found in the trunk of D4vd's Tesla this month in L.A. Once news broke of the grisly discovery, Damien recalls feeling shocked and saddened, and he regretted never getting a chance to say goodbye to Celeste.