So maybe the reason he cheated was because he lost sexual desire for you. If you want to fix your relationship, you should take care of yourself—focus on your appearance, stay fit, eat healthy, and try to make your sex life more exciting. You could also research different ways to improve intimacy to get ideas. Maybe that could help bring back your husband’s sexual interest in you, if you really want to work on the relationship.
But for me, it might be better if you just separate, because he already cheated on you and had sex with someone else. And in my opinion, no matter what the reason is, cheating is never justified.