When will I be the person I visualize?

They say if you can learn to do things just by practicing visualization. I visualize myself being so fun and easy going that I actually believe it. Until... someone phones during the last 20 minutes of the movie I'm watching and I get snippity. Then they ask if I want to have dinner and I get more snippity. Then we meet at the restaurant and they couldn't find my table so they sat at another one and I get REAL snippity. I can visualize each instance and practice saying the opposite of what I did say but why oh why isn't it be easier to be nice the first time. Why is it that the Grumpy McGrump part of my personality responds to every situation first?