The "Pose" star says after his HIV was first detected 14 years ago, shame and fear drove him to keep it a secret ... especially while trying to make it in an industry he says hasn't always been kind to him.

Porter, in an essay for The Hollywood Reporter, says ... "I was trying to have a life and a career, and I wasn’t certain I could if the wrong people knew. It would just be another way for people to discriminate against me in an already discriminatory profession. So I tried to think about it as little as I could. I tried to block it out."

The actor says people who needed to know about his diagnosis knew, but he hid it from everyone else -- even his mother. However, Billy says the past year of quarantine has taught him a lot and he began real trauma therapy to deal with his past ... and it led him to decide to reveal his truth.

Before that, Porter says he used Pray Tell -- the HIV-positive character he plays on "Pose," a role that won him an Emmy -- as his proxy. Billy explains, "I was able to say everything that I wanted to say through a surrogate," while acknowledging nobody involved with the show knew he was drawing from his own life.

In an emotional interview with Tamron Hall Wednesday, Billy revealed he used to question why he was able to survive ... and says he felt like his casting on "Pose" was a sign from God.

Now everyone knows, and Billy says that's liberating. He also points out that he's the healthiest he's ever been in his life and he's hopeful the stigma around HIV can become a thing of the past.