Hello! I Seem To Have Developed A Habit Of Shaking

Hello! I seem to have developed a habit of shaking my head. I am 55 and this started about 5 years ago. I have told 3 different doctors to check for Parkinsons and they all say I do not have it. I can stop the shaking anytime I think of it. However, it happens when I am not thinking of it, more and more often. The worst is when I am on my computer or smart phone or iPad, but it happens most of the time now my wife says. She says it is habit since she saw it come on.... but the doctor says it could be essential tremors. What do you think, and how can I stop it?