Without a mission statement clearly in your mind, your energy will be dissipated. A mission statement can change every few years so you always have to be reevaluating what your priorities are. Not having a clear idea of where you want to go and why will leave you floundering.
Writing a mission statement is not easy. One way to start is to write down everything that is important to you and make lists of order of importance. I like to do mine in my morning writing to look at it. Writing it down makes it easier to assess. Once I locked myself in a room till I thought of one. Its a good exercise and will help with any area of life. This is more than goal setting. This is direction setting.