People can say what they want of you. That is their prerogative. The more they underestimate you, the better. Do not be bothered by it. I know it might be hard the first few times, but once you get over the need to being liked and appreciated by others, the better it will be for you. Do not allow others to dictate how you feel about yourself. As long as you put in your best to your work, that is all that matters. You can only control what is at your end. Let them underestimate you. You keep on working hard. The result will speak for itself.