I think you're right. She had a sense of responsibility, from what I get in the various scenes we've seen her in, and feels the need to consider how she can provide for herself. The Campbells have been very generous with her, but I can't imagine she expects them to keep her with them forever. And living with her Highbury family, while possible, is not at all in the best financial interests of her aunt and great aunt.

So what's this thoughtful young woman to do? Sure, as mentioned by Louise above, she could hope for a suitable marriage. But we see from JA's novels that this is usually much easier said than done. On the other hand, she's qualified to be a governess and sees this as a prudent way to relieve her family's burden and provide for her own needs. Marriage may eventually come (as "poor Miss Taylor" happily discovered), but in the meantime Jane Fairfax needs to pursue plans for keeping body and soul together.