Re: i need major help
While your affair and its implications for your career and your marriage are one thing, his behavior is another--and his threat of violence against himself should have been taken quite seriously at the time. That's when you call the cops or insist he go to an ER (that's where the cops would take him, anyway), of course, once you have the weapon beyond his reach. His behavior was extremely manipulative, as is his threat of "telling on you." Take those threats away--insist he get mental health treatment or you leave. Also, can he actually prove you had an affair? If not, then I wouldn't worry too much about it. Chances are that people know he is very manipulative and they may see it for what it is: a spiteful attempt to embarrass and discredit you so you won't leave.
Without mental health tx, I don't see much help for your marriage. His behavior suggests some serious issues. Yours has been unwise and impulsive, too, so get some help yourself. You are both young, but not so young that impulsiveness is totally normal (esp. from a woman), and there is pretty much no age at which holding a gun to one's head is considered "normal" behavior.
Good luck. And thank you for your service!