Hard to say, but a few key pieces of information might lead you in the right direction. Is the inability to make progress project-specific? You can test this by doing something small on the side. Is it accompanied by an "ugh field"? Do you have non-programming-related signs of depression?
It's not project-specific. It's not repulsive so much as slippery - I happily begin working, but constantly lose focus. I was diagnosed with depression over a year ago, but I'm on meds and it's pretty much gone, and I don't have trouble focusing on things that don't require much insight.
P/S/A: There are single sentences which can create life-changing amounts of difference.