Many people who have low esteem feel that they are not entitled to have an opinion, because what they think and feel has no value. When somebody says something that you don't agree with, try saying so. There is no need to get angry or make a long speech: just say, "I don't agree."
You will find that the sky does not fall in when you say that. Often times, nothing happens at all. Sometimes somebody will ask you why. If that happens, limit yourself to saying what you think or feel about the question without turning it into a battle about who is right. People can have different opinions without one of them having to be right and the other wrong.