Successful freelancers need to charge at least double the hourly rate that they'd be happy earning doing full time work. (In many fields, it's more like 4 or 5x).
And they need to spend at least half their time getting better at their craft (and helping the market understand and appreciate what they do).
Your mileage may vary, but one sure route to becoming an unhappy freelancer is charging just enough and hoping that the low price will keep you busy all the time.
[If you're a freelancer with a career or marketing question, I'm recording a course on this topic and will be including reader questions as part of it. The form is open until tomorrow, Monday, at midnight. Thanks.]