Feeling upset and sad

So I found out last Friday, that my ex boyfriend who I dated for two years was cheating and messaging his ex let’s call her S the whole time we were together. He even was messaging her while I was pregnant talking about how it would’ve been fun to marry her. I found out that same Friday from his mom, that my ex also cheated on his last girlfriend before me, with the same ex S. His mom told me she doesn’t like the ex S and that she’s been popping in and out of his life for the last 4 years. It’s sad that he cheated on me while pregnant and his last girlfriend for someone he dated for only 6 months. I can’t help but think why does he keep running to her. What does this ex S, have that me and the other girl he dated before me doesn’t have. Mind you this man is 30 and he seems to not be happy in life and blames everyone for his life’s problems. He wasn’t happy with me nor his last girlfriend before me. Can anyone give me advice on how to move on. Please being recently single and a mother is hard and depressing. Thank you all.