Former Playmate Ashley Mattingly -- a domestic violence victim at the hands of Lane Garrison, who struggled with substance abuse -- is dead ... TMZ has learned.

Ashley's sister, Christy, and twin brother Billy tell us she took her own life Wednesday at her home in Austin, Texas and she left a suicide note.

We're told Ashley was found unresponsive Friday after a friend called the Austin Police Department for a wellness check after Ashley went radio silent. We're told police took the note as part of their investigation.

Ashley's family tells us she had been living in Austin for the past two years, and she was struggling with substance abuse. We're told she had demons and battled alcohol abuse, but she was trying to get her life back on track.

The shelter-in-place order in Texas did not help Ashley, her family says, even though Ashley recently adopted a 9-month-old golden retriever puppy to help with her loneliness. We're told the pup, Miss Jean, is now with Ashley's family.

As we reported ... Ashley, who was Playboy's Miss March in 2011, was charged with DUI back in 2016 after allegedly plowing her golf cart into 4 parked cars while she was wasted.

As you know ... Ashley was a victim of domestic violence. Lane, her ex-boyfriend, was convicted of domestic battery in 2012.

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The last time we saw Ashley, back in 2015, she seemed to be in good spirits.

Ashley's relatives also tell us they're thankful her friend called police for the wellness check ... and they hope her story can raise awareness about suicide.

Ashley was 33.