Sex, lies, and hate!

How do you get over someone cheating on you lying about it while telling you they love you whispering all the sweet nothings they used to but on the other hand telling everyone else that you're not together, their not sleeping with you and basically have nothing to do with you. Feel like you're the secret yet instead of the other chick he screwing???
So on the flipside you say we're going to work it out but now I don't trust you and my love for you has changed in a major way it affects me mentally on a daily basis and when I try to ignore it I just can't sometimes. I don't even like having sex with you because I know you share the same things with her. I look at you and it hurts I wonder why I am still with you? I feel stupid for accepting all the things that you have let me down about.
At what point does believing in someone become stupidity? 
With that being said I feel like a man is going to either accept the fact that he is the reason you feel this way, try to bring you some kind of peace with what he has done to you, do better and be better or he's just going to be what he is and at this point I don't know what you are 😕