The comedian shared her struggles in a new interview with People ... which she also details in her new memoir, "I Curse You With Joy," which drops in less than 2 weeks. One of the big takeaways in the book is that Tiffany says she's suffered eight miscarriages.

Tiffany has previously shared her fertility struggles, but now we're getting more of a sense of why -- as it turns out, she deals with endometriosis ... which means tissue grows outside the uterus, which can affect a woman's ability to conceive.

That's what's been happening to Tiff for years now. She says, "It's so f***ing devastating. Every time I find out I'm pregnant, I’m like, 'Don’t drink, don’t smoke, don’t do nothing.' Even if I don’t really want [the baby], I still try to give it a chance."

TH says she's experienced tremendous pain as a result of her endometriosis -- which she says was misdiagnosed at first. She explains, "I'm pretty sure the devil is real, because the first day of my period, no matter what, the devil goes into overdrive. I feel like my life gets turned upside down. I'm like, 'Am I dying?'"

Tiffany revealed she's currently celibate amid her health struggle ... but is still dating.