I know from times in the past where I've had problems getting things done is that there is some important issue that I'm not dealing with. It can be hard to complete small tasks like cleaning my room when there is a much large task that needs my attention. This can be caused by not knowing how to approach the larger issue or even being afraid to acknowledge it.
There has already been a lot of good advice in this thread but I'll add my thoughts as well. If there happens to be an issue that is overwhelming start by writing it down on a piece of paper and breaking it in to smaller pieces. Be aware of which pieces make you feel anxious and try and understand why and find a strategy to help deal with it. Thinking things through and writing them on paper does not mean you have to commit to anything but it definitely helps bring more awareness to the problem and hopefully give you more options.
Steve also has lots of good articles in the motivation section of his blog: Motivation