Motivation is a tricky thing. On one hand, some amount of motivation is required to accomplish anything in life, but on the other hand having too much of it can prevent you from succeeding as well.
There is nothing wrong with being excited when you first decide that you want to make a change in your life. That kind of motivation is powerful because it makes you want to act immediately toward accomplishing your goal. The problem is what happens when that excitement runs out. The human body and mind is powerful and capable of incredible things, but it has real limitations based on your individual strengths and weaknesses. These limitations are temporary and they can be pushed, but they cannot be ignored. Great change happens over a long period of time and only with consistent effort. That is why being too motivated can be fatal to your goals of success.
In highly motivated states of mind, people set unsustainable standards for themselves. For example, a person who wants to lose 50 pounds might set herself a goal to run thirty miles every day in order to lose the weight in a month. That kind of training will hospitalize the average person within a week. A more successful approach to a goal like that would be to start by runing half a mile a day and slowly building up to thirty miles over the course of several months (or years) depending on how your body reacts to the training. When you first start off with any kind of goal, you have no idea what your body and mind can handle. It is better to err on the side of caution and take things easy in the beginning until you get a better idea of how much more you can push yourself.
Yes, it might take you longer to accomplish your goals, and you may feel a little frustrated at your progress in the beginning, but it is better to do slow but consistent work that eventually pays off than it is to try to do everything all at once and quitting. Quitting is the number one cause of failure. Don't let something as silly as having too much motivation be the reason why you fail.
-Dustin Pak 2013