Hi Gillian,
It makes perfect sense that you’re worried about getting TOO focused on one subject matter (finance and billing) and, therefore, making it harder for yourself to move out of it. But I don’t think this is nearly as much of a barrier to getting a new job in a different field as you think it is! Yes, your main focus of this job is specialized in the finance world … but I’m confident that billing is NOT all you do all day long. You likely use LOTS of “soft skills” in addition to your technical knowledge. And those soft skills make you MUCH more appealing to other companies than you might think. Check out this article on why soft skills are so valuable and what constitutes a “soft skill.” Toward the bottom of the article, you’ll see two lists of soft skills (internal and external) — try jotting down a few examples of how you demonstrate these soft skills at work (and in your everyday life, too!), and look for jobs that value these. Focusing less on your technical skills and more on your soft skills will help you bridge the gap to a new kind of job in the future!