Am I an abusive friend?

I am generally a very nice person but I find myself constantly putting down a friend of mine. I am 30, she is 55. She is single, I have been married for a 6 years. There is a definite power struggle and I don't abuse her physically. I get upset when she doesn't do things my way and then I will treat her like she is an incompetent idiot. I can be VERY condescending. She tells me it bothers her, she threatens to end our friendship but I get too much of an endorphin surge from taking advantage of her this way to stop. Am I abusive? What do I do? It is a rush to act this way towards her but I want to stop :(