I know how it feels, that sucks.. best case scenario is he is polygamous and only has you and sam and he is an asshole. Worst case, he is only an asshole and could never have been trusted. The thing is you will notice you will never believe anything he says anyway, he can't prove negatives. At some point you will realize you have to keep trusting a person that broke your trust in the worst way possible. That takes strength and maybe it's naive, unlikely it pays of. Or you do the smart thing and move on. Sunk cost fallacy is a thing it feels like you're not ever gonna find that fit again and if you dump him it's all wasted ..it's terrible.. but I recommend moving on, there is so much more life ahead of you and you can find something much better. In my case the other person didn't know about me either so we got to be friends and took a 1 week vacation together to get it all out. But it looks like Leo knows you exist and are infact a gf who wants a monogamous relationship,the chance is low that he doesn't know and sam told him "oh we are an open relationship" or smth.. but you can be a better judge of that. So Leo is very likely an asshole too.

All my strength to you, you can do this.