What do I do about her??

My now fiance cheated on me and left me last June, and now we're back together but the other girl is pregnant, i'm in a different state than those two at the moment and I'm terrified that he's going to go back to her because she's having his kid (she's due in eight weeks) and i don't know what to do. i keep having these god awful dreams about the two of them and it's really frustrating because even though he tells me over and over again that he's not going back to her, it's hard for me to trust him because he's left me for her before, and he had no reason, what's to stop him from doing it again, and this time he HAS a reason. I don't know what to do or think or anything...help would be mucho appreciated. Thnx!