And then you don't.
It could be a fear of change; it could be a fear of improvement; it could be that sitting still and not doing any of it is just plain easier.
But you really, really want to! Kind of. You think.
The approach I tend to take with clients who really-want-to-I-think, or who are just plain stuck on the couch, is to take baby steps. Really tiny baby steps. I even have to laugh at myself sometimes when I catch myself saying to a client, "What would you think about thinking about..." making some type of change in his/her life. If diving into something is the hard part right now, get used to the idea of it, before you dive in. Think about what your life could be like if you were to take some real steps toward your goal. Think about your life could be like when you accomplish your goal. Think about why you have this goal. Think and think and think, and plant the idea into your head so hard that you want it, and sitting on the couch isn't comfortable anymore. Build some internal momentum to help you get started. Then when you can't sit still on it any longer: go! You have the plan in mind; you've been thinking about where you need to start; you've been developing an idea of what you want this to look like. So go for it!
A lot of times, doing the mental prep work for making a change in your life and getting yourself to the place that you're really, really ready to do it, is a big part of the battle. Once you start taking the steps to make it happen, the momentum happens pretty much organically.
What do you want to do or change that you keep putting off, just because? Start thinking about it. Create a picture of it in your head. That's it. Then see what sort of life the picture takes on for itself.