affair and gaslighting

my now ex-boyfriend has been having an affair for about four years at this point. He has refused to admit that it's an affair and has been trying to gaslight me into believing that nothing is going on between them and that it is all in my head. Ii layed out why I think it's not all in my head and he promised he wouldn't talk to her on four seperate occasions each time he would do it behind my back. Now he's open about it but will not leave her even if i threaten to leave. Both he and the affair partner have said they are just close friends, and my ex-boyfriend also says they just have a brother-sister bond, that she is his bestfriend. I know that isn't true, their affair is glaringly obvious, I just haven't wanted to believe it. He says he doesn't have sexual feelings for her, doubt this is true, as he can form sexual attractions to anyone, wether it be a model on instagram or a pornstar. I wouldn't be surprised if they've actually had sex or something. We have a bad relationship in general due to my resentment and refusal to get over or accept whats going on between them, another reason I dumped him. But he still acts like we're together, and when I don't engage, he threatens to kill himself and goes into disturbing details about how he'll brutally do so, how I'll be better off w/out him etc. He said he'd marry me in the future, but I know he doesn't have the commitment required. He has said that I'm the only thing that makes him happy, that nothing else can. I just want him to fall in love w/ someone else at this point so he doesn't have to rely on me anymore.