Single mommy question

I’m a ftm and of course i imagined everything much different then how it’s going now. Sadly, i don’t think me and my baby’s dad will be staying together much longer. he truly robbed me of what’s suppose to be the happiest time of my life and put me through hell. Anyways, for some reason I’m scared, I’m scared he’ll start another family with someone.. no idea why I’m scared cause if he sucked so much at keeping this one together why should i care? But for reason I’m just Imagining and keep thinking of him being the man i wanted him to be with someone else and them having a great pregnancy together and i can’t get it out of my mind. I want to just not care because honestly that would make this detachment thing so much more easier until i move out. Did anyone else have this fear of your ex partner having another kid and being with someone else?