Chrishell broke down telling co-star Mary Fitzgerald about exactly how she found out the "This Is Us" star wanted out of their marriage. It's pretty heartbreaking ... Chrishell choked back tears before revealing, "I found out because he texted me that we were filed [for divorce]."

The actress/real estate agent says at first she thought Justin was joking but pretty quickly reality set in ... adding that "45 minutes later the world knew." TMZ broke the story ... Justin filed for divorce in November 2019 after 2 years of marriage. She later strongly hinted she was blindsided by the news ... and now we know why.

Chrishell says she spoke to Justin after he dropped the bombshell on her but that was the end of their communication. She was clearly distraught ... saying even though they were only married for 2 years they were together for 6 years. She said if this is what Justin really wanted "there are better ways to go about it."

The Netflix hit "Selling Sunset" premieres season 3 beginning Friday ... and the season's storyline largely centers around the sudden end of Chrishell and Justin's marriage.

That's playing out pretty publicly -- Justin's moved on and has been photographed getting cozy with a new GF.