Heartbroken-LONG sorry

So my boyfriend and I had to go to trial today for custody of his daughter.  Basically the mother is very unstable but wants to move the daughter out of state with her for her job.  Bombshell hit me today that he and the ex slept together 6 months into our relationship (over a year ago).  He told me this morning an hour before the trial.  I spoke with his attorney before I testified and he said that he told my boyfriend to tell me about what happened in case it was brought up in the courtroom as an argument that our relationship isn't stable.  I feel angry, disgusted, heartbroken, and so many other things.  He's not the cheating kind of guy and it blows me away that he would let this hussy/manipulator seduce him when he's supposedly in love with me.  Now I'm questioning everything and my trust in him is gone