The excuses vary, but they are all bad. If I don't follow through, it's because I didn't want to follow through. For me, wanting means willing and able. Everything else prevents me from knowing myself. If there's something you feel that you wanted to do, but didn't, try restating it with 'wish' instead of 'want'. It's rough.
I know what you're saying, and its what keeps me from freaking out and being angry/sad when I don't follow through. However- and this is kind of what got me posting this thread- I have the irking feeling that I am lying to myself when I decide "eh, I don't want this." I might hate the process and all of a sudden I'm justifying the failure, from acheiving something incredible, by saying I didnt want it anyway-- when in reality, I just didn't want to suck it up and work.