Not to detract from the YC experience, but I think you can create this sort of rhythm independently, too. Demos in front of a <100% complimentary audience (i.e., not your cofounder/girlfriend/parents) can be enough to force you to develop many of the habits of a successful startup (fail fast, pivot, stretch yourself, listen to customers, etc., etc.) no matter where you are.

That discipline, coupled with an honest daily status report to my team, effectively ended ~10 years' worth of my own bad coding habits (procrastination, under-estimation, and insistence on clever/perfect/fast solutions where "good enough" was in fact enough) in about a year.

The extra pressure of a YC demo day may help increase your pace in the short term, but you eventually need to learn the discipline to show continual improvement without the pressure of a room full of VCs criticizing your every word.