Charlie Sheen revealed he is HIV positive Tuesday in an exclusive TODAY interview with Matt Lauer. "It's a hard three letters to absorb. It's a turning point in one's life," the 50-year-old actor said.Sheen said he made the announcement to stop a smear campaign and extortion efforts."I have to put a stop to this onslaught, this barrage of attacks and of sub-truths and very harmful and mercurial stories that are about me, that threaten the health of so many others that couldn't be further from the truth," he said.Sheen said he was diagnosed roughly four years ago but doesn't know how he contracted it."It started with what I thought was a series of crushing headaches," he said. "I thought I had a brain tumor. I thought it was over."He also insisted it was "impossible" that he knowingly transmitted the virus to others. While he admitted he has had unprotected sex with two people since the diagnosis, both were informed ahead of time and have been under the care of his doctor.Sheen said he's trusted the diagnosis with people he thought he trusted, but some people have extorted "millions" from him to keep the illness a secret."What people forget is that its money taken from my children," he said about the "shakedowns" he has experienced."I trusted them and they were deep in my inner circle, and I thought they could be helpful," he said. Instead, "my trust turned to their treason."But he said appearing on TODAY will help put a stop to those payout."That's my goal. That's not my only goal. I think I release myself from this prison today," he said.He said he hopes by going public with the diagnosis, it will help dispel the stigma of HIV."I have a responsibility now to better myself and to help a lot of other people and hopefully with what we're doing today others will come forward and say, thanks Charlie," he said.