Motivation

Why do we go through these phases of little to no motivation even though the destination does not change. Our purpose for being here does not change. Each day i think i am going to be productive today but lo and behold...i don't get much done. I have to prepare for my oral exam which I would like to complete in less then 2 weeks. I also have this other research project that i am a couple of months behind on. Getting started is the hardest thing and i am so easily distracted. Once i get started i can at least make some headway into being productive. So if you are reading this and have some suggestions about being motivated and staying motivated...then please share. its not the overall motivation, its the daily grind that the issue.