Ugh, consider this... you are new parents. You have a lot of hormonal surges and a new little human being to bond with and take care of. Many poly people take a break from engaging in other relationships when there is a new baby in their lives.
Instead of placing blame on yourself and thinking you are not handling your "triggers" well enough, consider the possibility that your irritation rises up because this simply is not the right time for him to be focused on having liaisons with other women. You and your baby need him!
You cannot be left (abandoned?) to parent alone if you have a partner. Is he stepping up and doing what he needs to do to share in responsibilities at home? If so, then, okay, maybe have one day a week for a date with someone else, BUT if pursuing other women is an escape from the scary responsibilities of being a new dad and he's leaving you high and dry to do it all by yourself, then he is being a total cad and you should not tolerate that!
You two need to get down to the nitty-gritty in your communication and come together as two new caregivers of a child.