The best sentence I´ve found so far to summarize the daily struggle of solo entrepreneurs (specially when they have a full-time day job) is the following:
“We must all suffer one of two things: the pain of discipline or the pain of regret and disappointment.” by Jim Rohn
Some days finding the energy to keep going with your entrepreneur project is a real challenge but the alternative is even less enticing, take a short rest today but feel bad for the rest of the week! Completely up to you 
Do you want to share other similar quotes you´ve found inspiring on this topic?
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Regret the headline (and the first sentence).
Thanks, typos fixed!
Related to this, read also this recent great post on “it takes hard work” http://joel.is/post/16350861743/it-takes-hard-work-do-the-hard-work