My fiancé cheated,,, with a man...

So, I’m honestly devastated but so god damn confused on what to do!! I’m almost 16 weeks pregnant, with our first baby, and our relationship had been going pretty well. Then all of a sudden he started drinking again, and had been doing drugs behind my back.. One day I was taking care of my sick mother before a major brain surgery, and he calls me crying saying he cheated! He lied and said he cheated with a random girl, but I could tell he was lying so I kept asking him what the truth was.. then he spilled the beans.. he hooked up with a guy he met down town on a drunken bender. So now I’m at a loss. He says he is going to stop drinking, and get back on the wagon, but I don’t know if I’ll ever be able to trust him again. I have no idea what I should do in this situation.