My worthwhile project was when I really stepped outside of my comfort zone to lead a complete redesign / overhaul of a datacenter over the course of 9 months.
I had the knowledge, and the desire, but probably lacked confidence in my ability to be the lead design engineer for such a large project. I had always worked in a NOC/operations role where I excelled at any task that was put in front of me, but I had a lot of questions, always.
Fast forward a few years - I took a leap and went for it. I got hired on for a 9 month contract to replace racks and racks of networking equipment, an entire replacement of everything layer3 and layer2 in this datacenter, complete with remote monitoring/management facilities.
I went full bore - I didn't sleep much. I ran the systems I was working on at home in my lab, virtually, and basically taught myself Juniper and Arista from the ground up - having only worked on Cisco devices previously.
Long story short, the conversion was finished ahead of schedule...but the contract had me there for a while longer, so I got tasked with a bunch of crazy 'stuff' - things that I've never touched. Load balancers, traffic shapers, linux servers (that actually did 'stuff') - just a ton of stuff to learn, understand, plan, and implement. Around this time is when I also got my introduction to the Python language (but that's more of a way to make things simpler to me, rather than what actually pays the bills).
It was through those 9 months, that I became so confident in myself and my abilities. I no longer doubted myself. I no longer thought that my questions were stupid. Because of that, I flourished. I've been in a senior network engineering role ever since...and finally feel like myself. It's great. Literally changed my outlook on life.