I don’t strictly agree with everything Walk Kania writes here, but in general, it rings true:
If I was to add a single comment, I might say something that Mr. Kania takes for granted: Remember that you are in business. Most freelance science writers I have known who have chosen to stop science writing have plenty enough scientific background and accuracy (although God help you if you don’t), and sufficient quality of writing (when pitched to the correct market), but terrible business skills. Marketing, maintaining current clients and regularly chasing new clients, as well as keeping track of time and overhead and invoices (and paid invoices) and taxes are all part of the job. Not the glamorous part, but a necessary part.
If that doesn’t break your will to be a freelance science writer, just spend a couple days looking for affordable family health insurance for the self-employed.