My wife has asked me to promise never to practise polygyny, but I feel as if I am not trusted. What should I do?

That request usually comes from a fear of loss, not from an accusation that you are a bad person. Instead of taking offence, try to hear the feeling behind the request: she wants to feel safe. You can reassure her by stating your intention and commitment now, while being honest that the future belongs to Allah. Avoid promises you yourself are not sure you can keep, because a broken promise hurts more. Focus on building trust through daily treatment that settles her heart. If she wants something legally binding, that needs to be discussed with a religious officer, not decided in an emotional state. See rights of husband and wife.