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What I'm gonna tell you only works if you're making software for a living.
If you're stuck with a problem and can't get past it, switch to another project right away.
Get some traction there, keep the flow going. Switch back when you're ready, you'll probably see something you didn't 15 minutes ago.
This is the rapid version of "sleeping on it". Anyone who came back to a project the next day, fixes something and then think "this is so obvious, why didn't I think of that yesterday?" knows what I'm talking about.
Again... one of those things that only happen in my business. Don't let nobody tell you writing two books at once is a good idea.