14 weeks pregnant and Fiancé just told me he’s in love with my best friend

The weekend of the 14th was just a normal weekend until my fiancé dropped a bombshell. We were fighting about something trivial, next thing I know he’s crying saying he thinks we should break up. We’ve been together 6 years and other than that fight he’s never said anything about being unhappy. I asked him why repeatedly and he finally admitted that he was in love with my best (and basically only) friend and that I’ve pressured him into everything. Her and I have been friends, no, sisters since grade 8. She helped him pick out my engagement ring, was there for me when I had a miscarriage in December, was supposed to be my maid of honour and came to special occasions with my family. I was absolutely shocked and devastated. I asked him to work through things with me but he insisted that he wants to be with her. I asked him how long and he said they’ve been talking about it for 5 months. I’m only 14 weeks pregnant. We own a house, we have a dog, cats and a baby on the way...I just don’t understand how he could do this. He finally agreed after spending some time apart that we should work together and try but he won’t stop talking to my best friend who has blocked me on all social media and only talked to me once on the phone to tell me that she loves him and doesn’t know how it happened (she didn’t even apologize and had a bitter attitude). He’s here but I feel so empty. I just don’t know what to do. I love him with all my heart, we’ve been together since I was 15. When I asked him why he would choose to make a child with me he said that he didn’t want to hurt me and didn’t think it would happen. I’m just so lost, I need some advice from people who aren’t close to the situation. He says he wants the baby more than anything and wants to take it day by day, but where does that leave me? I’m sorry for the long post, thanks for reading and commenting if you do.