How do you move on?

We've been dating for a year in a half. I have done everything I was suppose to. This girl has been dating my boyfriend and hers, she's pregnant with her boyfriends kid. I'm pregnant with mine. She is 19, I'm 21, his 21. She and my boyfriend were dating behind my back for two weeks I caught them, because she jumped right on him after we got into a fight about him lying to me and being with her. She was spending the night I got worse. I asked them both to stay away till I got my feeling sorted out, because they wanted to stay friends. I hacked his snapchat they both broke what asked, they both spent the night at either houses, though no sex just cuddling. I screamed, cried, begged. She says she has nothing to do with it, but she knew we were dating and still. I guess my question is he wants to fix it, but in the back I keep thinking he'll go back to her the second I leave. I hate checking his phone or stripping his freedom, I hate it. Yet I don't know how to explain to him or her this is wrong. She I qoute said 'she ruined us' she really thinks I'm the reason at fault her one month relationship with my boyfriend, who was taken and has been taken for two years almost, was my fault for telling her I was pregnant and I've been fucking him not 24hours after he left with her.