@poetryslam, I am so sorry for the trauma that your H has inflicted on you, his Wife who has given so abundantly to him and your children. You have been in an unequal, abusive relationship.
As I feared, he has indeed been investing in infidelity and defiling your marriage and family for years while romancing his girlfriend. It takes an especially cold and unempathetic man to cheat on his pregnant Wife. Neither of these devious snakes have an ounce of decency or integrity.
He could have shut down this affair way back when you expressed discomfort after DD1 was born, but instead he kept stealing your agency and consent for the thrill of his double life. He saw how upset you were about the holiday lights in December, but paid lip service to being remorseful and continued cheating and prioritizing OW. She likely spilled the beans about their excursion because she was jealous of your pregnancy and wanted to wound you.
In addition to H’s long-term infidelity, he dared to bring into your home OW’s gift which contains intimate symbols of their connection. Be aware that his crocodile tears and begging are performative dramatics meant to manipulate you. He’s bricking it because he doesn’t want to lose his professional/personal reputation, home comforts, or part of his income. You would be very foolish to believe anything he says. You now know what he is capable of.
I agree with others that after all this time they are physically involved, so you need to get an STD test. Also, please inform OW’s husband of her faithless behavior so he too can make decisions based on truths.
@poetryslam, I hope you have a loving support system surrounding you as you process your shock, pain and grief. Enduring your abuser’s presence will likely exacerbate your pain, so I suggest that you send him away for now as a sharp consequence and to give you space.
Such a heinous betrayal would spell the end for me, but should you decide to reconcile, he would need to comply with your recovery requirements, including changing jobs, going NC with OW, providing full transparency with devices, and digging deep in IC to examine his deeply dysfunctional flaws that enabled his adultery. Be aware that therapists advise it can take 2-5 years for recovery, and that is when the betrayer is fully committed to moving mountains.