advice please!!!!

I am constantly thinking about my fiancées ex's! I know everyone is going to say just stop it but being pregnant doesn't help. I am 15 years younger than him n all his ex's were his age or older so I should have nothing to worry about but it constantly bothers me. I have no idea why. I'm the first girl he has ever bought a ring for n the only girl he ever even thought about having a baby with. please any advice would help! I'm going nuts!!!!