tired of the games

my bf and i have been engaged for a year now and we r both 28. everything was great until i started noticing how he's always holding back with me in bed, like there's this wall between us that he never lets down. so last weekend when we were at his parents' place visiting them, we got into an argument cuz they mentioned getting pregnant soon as part of our future plans and it triggered him to freak out a bit. i kinda snapped then and asked him straight up if he was still in love with his ex-girlfriend or if something else was going on. we argued and he said it's not that but there is this other thing, which confused me even more. turns out he secretly started dating a coworker about 6 months ago and had been hooking up behind my back all this time. i feel so betrayed and now we're both trying to figure out what the future holds cus how do you stay with someone who's cheating on u