People call me a 'husband-stealer' because I am a second wife. How do I cope?

That label is unfair if your marriage is valid and was entered into the right way. Allah forbids mocking people and calling them by offensive nicknames. At the same time, understand that the anger often comes from the pain of the first wife and her family. Do not respond on social media or argue in public; dignified silence is usually more effective. Show through your conduct that you mean no harm to anyone. If the harassment becomes extreme — threats, slander or attacks — you can seek legal protection. See counselling.