The type of challenges you want are the ones that your brain shies away from - the ones that cause you intense anxiety, like you'll feel the world will end if you keep doing them and you'll be left totally exposed. (Obviously, assuming that there's no rational reason for the world to
actually end.)
So for me - the first major challenge was rewriting the database & website for FictionAlley.org in college. The reason it was challenging was that up until then I'd flaked on every project that I couldn't see the end of. I was great at stuff that could be accomplished in a few hours, but when it came to stuff that took sustained effort over months, I'd start to panic that maybe it wasn't worth the effort after all and what if it failed?
My second major project was Write Yourself a Scheme in 48 Hours, where there was this voice running through my head saying "Who are you to think that you can teach Haskell to the web? You barely know it yourself." It was a fear of being judged publicly.
The third was redesigning the Google Search Results Page in 2010. And that was scary for similar reasons - every whole-page redesign up until then had failed, and we were putting ourselves out there in front of literally millions of users. Luckily I had lots of support from my coworkers and senior management, and it wasn't me leading the project.
The fourth was Gumbo, which was scary because it was ambitious (a full implementation of the HTML5 parsing algorithm), and I was solely responsible for it with nobody backing me up. I'd identified a problem, no organization was supporting me, and I'd embarked on a multi-year 20% project to deliver it.
It's not just technical stuff either; you can become more determined by exercising the parts of yourself that you'd assumed are no good. For example, many geeks are absolutely terrified about talking to women. Or to strangers in general. The 2 years I spent dating, which eventually led to finding my girlfriend, were as educational and character-building as any software project I did.
Flossing every night will probably not cut it unless you have a phobia about flossing.