Taxes and time management.
Taxes — setting that money aside and not touching it until it’s time to make the next quarterly estimated payment. It’s brutal. I didn’t do very well at it last year. I’m doing better this year.
Time management — I left a full-time day job in 2012 to work on comics full-time. After several years of working 17-hour days so I could build up my career enough to support switching to full-time, I now find myself working 17-hour days so I never have to go back!
The irony, though? Time management was way easier when I had to use my time more ruthlessly. Back then, I had only a finite amount of time between my family responsibilities (wife, kids, house, etc.) and my day job. I did a 5-day-a-week comic strip, wrote books, maintained websites, did side projects and much more in those precious hours because I had to. Today, I have a much more relaxed schedule. But that has proven waaaaaaay more difficult to navigate!