Does stopping hard tasks midway through negatively affect your chances of success? When a person has embarked on an undertaking that is both unpleasant and important (a good example could be a 12 step drug treatment program), lets say they make progress, and then stall before completion. 

Is the the person significantly less likely to be successful as a result of the break in their momentum?
Here are some thoughts on potentially important moderating factors for any such basic effect: 


Whether stalling is voluntary
Whether there are social or environmental reasons for stalling
How long one stalls
Whether one has made a complete break from the undertaking

Whether one turns in the meantime to related matters that might facilitate or conflict with it 



I'm not implying you have to narrow your question down to get a general answer, but the general answer might not be a noticeably influential factor in specific subsets of the general question. For instance, going to sleep before trying to finish something one has been working on all day might replenish momentum the next day; this is a short, voluntary stall that probably addresses a particular reason for stalling (e.g., fatigue, or maybe needing daylight or businesses to be open to make progress), and people consolidate experiences and replenish energy resources during sleep, so sleep is a facilitating matter to turn to in the meantime. 

In contrast, stalling for months because of setbacks in one's personal life might better represent the kind of scenario you have in mind that would more likely prove detrimental. For example, say you're working on a research project, but happen to be living in an apartment you share with a crazy roommate. If your roommate suddenly becomes so disruptive that you have to find new housing, negotiate a breach of your lease, move out, and settle in to your new spot before you can get back to work a couple months later, your new rate of progress might be slower at first than when you left off. This stall is essentially involuntary, due to disruptive social influences in one's environment, long enough to potentially result in forgetting some aspects of how you were doing your work (which could require refamiliarizing with even your own organizational structure! My former advisor warned me about keeping detailed notes for this reason), and it's a complex enough reason to stall in the meantime that you might stop thinking about your research entirely, and have to focus on something completely unrelated...Then again, the result still might facilitate your rate of progress if your research was being hampered by living with that crazy roommate, and your new roommates aren't equally crazy.  (Based on a true story! :)

I suppose the implications for your question are that moderating factors like these may overwhelm any basic, duration-independent effect of stalling. Again, this is not to say the basic effect hasn't been studied, much less that it can't be...but my recent dissertation was on goal progress, and I haven't yet come across any sufficiently sophisticated research to address your basic issue, whereas there's plenty of research out there on relationships between progress and goal characteristics like environmental support and goal conflict, including a little bit of my own research. That being said, I'd love to see any research others might be able to contribute to this discussion, as I do find the basic issue interesting!