cancelling wedding?

I don't know what to do 😫 I'm supposed to marry my SO march 25th. That's only 3 weeks away. The wedding was planned for later this year but my future in laws pressured us to get married before the baby comes so we agreed on march. However, I found out my ex (also my best friend) who's never moved on from me and always been there took a girl or on a date. When my sister told me that I had a break down and threw up I was so upset. And I spoke to him, he said he wants to be with me and would help raise my child even though it's not his. The ex isn't the only reason I'd cancel the wedding though. My SO is so arrogant and selfish. He makes us split al of our costs. I've paid almost $2k on this wedding and he's paid $400. He's still up his parents butt.  He's cheap as can be. He talks down to me like a dog and we argue all the time. Honestly, I probably wouldn't have married him if I never got pregnant. We weren't trying to get pregnant, inwas in birth control. I'm so excited for this child and wil love it so much but I wish it wasn't his. I'd feel like a terrible person if I canceled a wedding 2-3 weeks before when 50 people have plans on coming and things have been bought. I also bekueve in trying to make it work with a child's father for the sake of the baby. I just honestly don't know if my pregnancy hormones are getting the way of me or if I'm making a mistake. I wish I had clear answers. I've prayed about it but no answers are clear to me. Any advice is welcomed.