My co-wife is much younger than me and I feel less confident. How do I manage this feeling?

Feeling less confident is human, but your worth is not measured by age or looks; it is measured by your faith and your role. The Prophet ﷺ once said “Your mother is jealous” (Sahih al-Bukhari 5225), showing that jealousy is a natural feeling that even noble women have experienced. Do not compare yourself with your co-wife; focus on looking after your health, your worship and your own relationship with your husband. Start by taking good care of yourself, not to compete, but for your own wellbeing. If the low self-esteem persists, talk to someone you trust or a counsellor. Age brings a maturity that is priceless. See counselling.