"Summer House" star KJ Dillard needed mental health treatment after the most recent season of the hit reality show ... revealing he was hospitalized shortly after filming wrapped.

The reality TV star made the shocking revelation during the season 10 reunion ... telling Andy Cohen he went to the hospital for about a week due to self-harm.

Dillard says doctors diagnosed him with Borderline Personality Disorder ... a mental illness that "severely impacts a person's ability to regulate their emotions" ... affecting feelings about themselves and negatively impacting relationships they have with others -- according to the National Institute of Mental Health.

He shouted out both Ciara Miller and Mia Calabrese for showing up to support him ... revealing the ladies sat at his bedside every day during his hospitalization.

Dillard admits he never dealt with his heavy issues head-on before ... choosing to run away from his problems instead. He says he's grateful to be alive -- because he very well could've died.

As Dillard began to cry, Jesse Solomon hugged him tightly ... and praised Dillard for telling his difficult story.

While a vast majority of the reunion's first part focused on the shocking romance between Amanda Batula and West Wilson -- including the pair maintaining they didn't sleep together until after dropping a joint statement announcing their relationship on March 31, despite kissing for the first time the previous month.

Part 2 of the shocking reunion comes out Tuesday, June 2 ... and Part 3 drops June 9.

If you or someone you know is struggling or in crisis, help is available. Call or text 988 or chat 988lifeline.org.