I have been freelancing for two years only, and I spent a big part of that period learning, writing and speaking at conferences. So, a lot of the great moments and feedback I got were from writing and speaking, besides developing code.
In 2014, writing became a big part of my career. I spent that year mostly writing a CSS Reference for Codrops. The reference is one of my biggest projects, and one of the projects I got the most amount of positive feedback on. Great feedback to my conference talks, especially the first one, and to my writing is what keeps me going. I improve on what I do based on the support and criticism I get.
I got my first freelance (development) client project two years ago. It was also my most challenging so I got to learn a lot from it to shape the rest of my freelancing life. Back then, I didn't have any experience dealing with clients, I didn't know how much to charge for my work and time, I didn't know what limits to set, and most importantly what my clients can and can't expect from me. I overworked, to say the least, and ended up having to quit the job with blood-red eyes, a stiff injured back, and a bleeding nose; that's what working 16 hours a day for two weeks did to me. I reached a point where it was either my health or the job, so I chose my health.
As a result, my following jobs were all very different. I learned how to organise my time and what limits to set and when. Reading a lot of articles about freelancing from experienced designers and developers also helped.
My second client was a Swiss design agency; I worked with them several times over the course of 6 months; they were a happy client and I was a happy and healthier developer. And have been doing this ever since.