My heart finds it hard to accept having a co-wife even though outwardly I have accepted it. How do I make peace with it?

True contentment grows slowly, and it is all right if your heart is still struggling while you keep trying. Start by separating what is within your control, namely your deeds and attitude, from what Allah has already decreed. Your provision and your share have been set by Allah, and they do not shrink because of a co-wife's presence. Fill your heart with worship, prayer and strengthening relationships, not with comparisons. Give yourself time, and seek a counsellor's support if sadness has you in its grip. Making peace with the decree brings a calm that fairness in the rota alone cannot give. See mental health.