Can a husband who discovered his wife's infidelity after they were subjected to a spell of separation marry another, despite his wife's repentance and pregnancy, under the pretext that he cannot forget what happened?

If you repent sincerely from your sin, Allah accepts repentance and forgives misdeeds, and your husband should overlook what has passed. If your husband does not forgive, that does not harm you if your repentance is sincere and purely for Allah. If your husband insists on marrying another woman, there is no blame on him if he deals justly between you, and you must fear Allah and not fall short in His rights. A Muslim must be keen to reside in Muslim lands to avoid the dangers of disbelievers' lands to religion and morals.