Lili Reinhart ’s keeping it real ... revealing she was finally told she has endometriosis after undergoing surgery -- a hard-won answer that came only after years of doctors brushing her off.

In an IG post Thursday, the actress shared pics from the hospital after her laparoscopic surgery ... explaining she’d seen multiple urologists, gynecologists, and hit up three hospitals, yet not one doctor brought up endometriosis. Instead, she was told last year she had interstitial cystitis -- and there was no cure.

It wasn’t until she saw two different pelvic floor therapists that endometriosis was even floated as a possible cause of her pain -- which pushed her to demand an MRI -- finally leading to the real diagnosis.

She says she’s just glad she listened to her gut ... adding with endomet

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