I accidentally revealed private information to my boyfriend's roommate's ex
Could you explain to me what kind of information you revealed to his ex?
The information I accidentally revealed to her about my ex-roommate was that he was a sex addict.
Partners are expected to keep secretes told to one another in private. You should be upfront and honest with him before he finds out on his own. He may be more willing to forgive you if he finds out it was done on accident.
I was never honest with him. I am a liar.
It doesn't sound like this was an accident, was it? You shouldn't reveal things that were told to you in confidence-- most people will think that you're being a bad person.