What to do about this situation?

So I am currently 37 weeks, 4 days pregnant, baby is due at any time now, Monday, my significant other left me to go be with a married woman. He went to stay with his mother, she lives closer to the woman and her husband's house. The married woman is his best friend's aunt. She keeps harrassing me on facebook, I can block her and somehow I wake up the next morning with another message in my inbox, telling me how pitiful I am. I have told him about the messages, but he claims it's nothing he can do. Isn't it enough that I have to go through parenthood alone, again? It's clear that he won't be there for the delivery or sign her birth certificate, his father works in chid custody and child support, so if I try to pursue that, I will most likely get shot down or slapped in the face again. My mind tells me that it's okay and I can do this alone, but my heart really aches for my daughter and my 3 year old son. Should I just take it as a loss, or should I just pursue it only to get slapped in the face again?