In addition to "I had sex w my boss"

Everyone is saying I should keep it to myself and just move on. But it's so much harder than that... He wants to take me out Monday night and for me to spend the night, but I've been talking to her and I know that they're still together kind of. I can't say that to him though because he's not supposed to know we have each others numbers or that we've been talking. But if I tell her what's going on she'll confront him about it and he'll know it was me because no one else knows. 
Also I'm supposed to work for his brother in June as a nanny, so I can't just move on or get a restraining order.
I know this is a complicated and stupid situation but it's been stressing me out soooo much and I have no one to go to. If someone could even leave their number or something so I had someone to talk to, that would be nice