How can a wife deal with a husband who is having a relationship with another woman for the sake of "business interests," despite her attempts to end this relationship amicably and firmly, and what is the effect of that on her psyche and pregnancy?

What you have mentioned about your husband is true, and there is no doubt that he has committed a grave transgression.

We advise you to be patient, to engage frequently in remembrance of Allah (dhikr) and supplication (du'a), to adorn yourself for your husband, and to be a good wife to him. Continue to advise him, and you may threaten to inform his family. If he does not desist, you have the right to seek a divorce due to his transgression, but do not rush before discerning the benefit [of such an action].