Thomas said he contemplated driving off a cliff as he did not want it to be known he was suffering from the virus.
The 45-year-old, who played 100 times for his country, also revealed that blackmailers threatened to reveal his diagnosis before he made the announcement.
"I've been living with this secret for years," he said.
"I've felt shame and keeping such a big secret has taken its toll.
"I had a fear people would judge me and treat me like a leper because of a lack of knowledge. I was in a dark place, feeling suicidal. I thought about driving off a cliff.
"To me, wanting to die was just a natural thought and felt like the easier way out, but you have to confront things.
"And having a strong support system and the personal strength and experience of overcoming those emotions got me through it.
"Many people live in fear and shame of having HIV, but I refuse to be one of them now. We need to break the stigma once and for all. I'm speaking out because I want to help others and make a difference."
Thomas added in a video posted on his Twitter account: "I want to share my secret with you. Why? Because it's mine to tell you. Not the evils that made my life hell, threatening to tell you before I do.
"And because I believe in you and I trust you. I'm living with HIV. Now you have that information, that makes me extremely vulnerable but it does not make me weak.
"Now even though I've been forced to tell you this, I choose to fight, to educate and to break the stigma around this subject."