Here's what went down ... after TMZ broke the story the reality star was entering treatment to work on his mental health struggles, Rachel went on Instagram and wished Jax a speedy recovery -- hoping he didn't face the same "mockery" she did during her own stint away.

As Rachel put it .... her former "Vanderpump Rules" costar's decision to seek help highlighted "the urgent need for mental healthcare and accessible treatment for reality tv stars."

It didn't take long for fans to unload on Rachel and her message to Jax ... they flooded the comments section with claims she was making Jax's news about herself. Many painted the ex-beauty queen as "odd" and self-involved ... though, a few fans came to her defense and said she clearly meant well.

As TMZ previously reported ... after her affair with costar Tom Sandoval went public, Rachel checked herself into The Meadows in Arizona -- where she underwent intensive therapy for months. She ultimately chose not to return to 'VPR' ... and instead launched her own podcast, "Rachel Goes Rogue."

As for Jax, he hinted at his struggles before he checked himself into in-patient treatment. In fact, on the podcast he shares with his estranged wife, Brittany Cartwright, the reality star admitted he had a breakdown after meeting a female doctor at his new sports bar.

He admitted to struggling with serious anger issues ... something he didn't want his young son, Cruz, to see.