Here's my perspective over the years of working. For as long as you work for someone, you'll always be "replaceable". Setting yourself apart from the rest is vital considering the competitiveness for work these days. It sounds like you are "okay" with your job, but not "hate" it, sort of in the middle. Also, how many people do you know that you can truly say they "love" their job/work, the type that can do this 7 days a week with no complaints? I think for most people, it's hard to say they "love" their job/work; hence, for you to feel/think this way with your uncertanties, I think it's normal.
In terms of going back to school, what are you defining as "old"? I am 29 and taking online courses in addition to full time work
. I can't vouch for finance as I am from an accounting background, I have heard finance, have to bust a crapload of work hours along with the competitiveness due to a lot of people wanting to get into that industry.