Post break up assumptions

I assume he will move on and live happily ever after and I will end up alone and miserable. We were together for 6 years and engaged for two. My fear is he's going to sleep with someone and she will get pregnant 😢 he ended things cause of a joint decision on an abortion 5 years ago and we were recently trying to get pregnant for a year, no luck. He thinks the problem was him but he refused to get checked. How do I stop assuming the worst about everything?😢