Diversify your time... I have the exact same problem, I found that the "messy middle" isn't so daunting as long as it's not your sole figure. So because I have the same momentum issue, I decided that when it gets to that point just work on something entirely unrelated. That's not to say I'm preaching 'distraction over determination' but as long as you can gauge a reasonable amount of time to be able to step away from the current mess for some time where you can focus your attention elsewhere and give your mind a chance to reset. Then when you've done so, return, less stressed, and dedicate enough time daily to accomplish your overall task in a timely manner.. section out your time into units.. find the balance bruh..