Jason Oppenheim is reaching out to his ex, Chrishell Stause, amid her difficult breakup ... telling his ex he's here if she needs anything.

The real estate mogul and reality star posted a comment on Chrishell's Instagram video where she revealed the news ... telling her he was sorry to hear about the breakup.

He wrote, "You're both beautiful people and sometimes things just don't work out, but it certainly doesn't mean anyone failed. There's probably nothing anyone can say to make you feel better right now, it might just take some time to process. But please know that you're very loved and I'm always there for you no matter what."

Remember ... Jason and Chrishell were together for about 7 months -- a couple of them secret -- back in 2021, and he was her last serious relationship before she got together with G Flip.

Despite their past, Jason's always been super supportive of their relationship ... at one point calling them an "inspiring" pair.

We broke the story ... G Flip broke up with Chrishell -- though they said they would always cherish their relationship with Chrishell.

For her part, Chrishell shed tears in the video announcing the breakup ... while admitting she's still in love with G -- and feels like a failure that they didn't go the distance.