Not Me!

I believe I am usually a kind person and try not to get upset too much.  I try to not cuss ever and for the most part believe that is true.  However, I have found that one of the people in my house is able to get under my skin so much that I become a person I hate when things get difficult with her.  I wonder if it is a test of my patience and ability to deal with the stubbornness of youth.  She is just half my age and maybe that is part of the problem.  I know that I have not experienced everything in life, but I do have some experience, and when it is negated and overlooked, it really bothers me... how is it possible to relate well to those who portray an attitude that they know more than you when you know that is not true!  I am an intelligent woman.  I have a bachelors and masters degree.  I am working on a second masters degree. I  have lived overseas for almost five years and have lived life.  I have lost people close to me.  I have been molested... I have survived everything I have encountered and grown from it.  Why does this one girl have the ability to get to me?
If anyone has any ideas of how to develop more patience and understanding in this situation, I need help!  I really hate who I become. I don't want to be that person.  How do I change it?