Why am i so scared/worried?

I’ve been with my daughters father for a few years now & he has cheated on me so i have a right to feel the way i do. Unfortunately I’m in no position to jus get up and leave him.. otherwise i would have i have no family to stay with and no real friends I’d trust having a newborn baby around. So I’m with him until i finish school & start my career. Anyways I’m so scared all day ever day he’ll cheat on me & get a girl pregnant.. i just can’t get over this fear no matter how much he reassures me he doesn’t want another kid or any other woman... but i wouldn’t put it past him.. i need advice on how to get over this fear. Even if i was the leave him and move on it would still hurt me if he was to have another child with someone else... any advice on how i can just let these thoughts & feelings go and not give a fuck?