I feel I am not good enough because my husband married another. Is it my fault?

Your husband's second marriage does not necessarily mean you have failed as a wife. Men take another wife for many reasons — some reasonable and some not — and it is not a measure of your worth before Allah. A woman's honour is measured by her piety, not by whether she is the only wife. Do not blame yourself excessively, but if there are things in the relationship that genuinely can be improved, discuss them honestly. Look after your mental health: talk to someone you trust, keep up your routine of worship, and get professional counselling if needed.