The MTV reality star and blogger, 34, had previously gone to extraordinary lengths to freeze her eggs in hopes of someday experiencing pregnancy and childbirth.
But when she was diagnosed with cancer for the third time, that dream was lost.
"I fought for it so much, and I wasn't allowed to have it," she tells PEOPLE. "I'm not pitying myself, but there's a mind-set that you can do anything, and that's what I'd always believed."
She adds: "I would love to be pregnant; I would love to feel a life growing inside me, but I know that's not an option anymore."
Instead, Brown, who is currently single, says when the time comes to start her family, she'll focus on an alternate route to motherhood: surrogacy.
"My big goal in life is to prove people wrong," says the activist, whose friends launched a support page for her on MedGift, the registry she founded for those suffering from any illness. "You think I'm out? Watch: I'm going to get healthy, and I'm going to have my family and my dream again."