Constantly feel like he's cheating!

Can anyone help me? I'm at my wits end. I constantly feel like my other half is cheating on me or talking to other people. I have absolutely no reason to currently feel like this.

I am 27 weeks pregnant and in my previous pregnancy my ex cheated on me, I can't help but think this is the reason i feel this way.

I have very vivid dreams of him cheating on me and then I'm in a foul mood for the rest of the day because I can't get it out of my mind.

I really want to stop feeling this way but how do I do it?!

Thanks in advance