I'm so torn: Prenatal depression & Relationship issues

Throughout my pregnancy, I've just been sad and I feel so alone. Idk what to do.. Months ago, I found out my fiancé cheated on me (bc we're in a ldr) and I can't get over it. I gave him another chance and I know and can tell that he has been trying so hard to change/show me that he's a different man but I think about him having sex with that girl everyday and I can't get it out of my head no matter how hard I try to. I love him dearly regardless of what happened but I'm so torn and I feel hopeless. Don't know what to do. Help please... :(