Honestly, I'm only 4 years older than you. I did an arts degree am completely and totally unrelated to the career I presumed I'd fall into (and have done).
What bothers me about your post, is that you say you're 26 and no and are successful, don't find your work a challenge, and haven't met your potential- you're clearly bright and interested in progressing... why do you need your parents, Mumsnet or anyone (how experienced/qualified are careers councillors?) to give you advice on what to do?
I think had my parents said to me at 18 "why on earth are you studying ancient languages" (which I chose) "that leads to no jobs...except teaching those languages", I'd have just shrugged and ignored her as I found Latin and Greek so interesting I studied it anyway.
You're clearly intelligent- but you come across as potentially lazy - wanting someone to give you the answer to a quick fix to be super successful. The internet can help, talking to people- your friends, managers, peers, customers about their perceived skills of yours or you failings. Research, find what you need to work on to get where you need to be to feel satisfied, learn, take risks and try.
My husband is huuugely ambitious and is always after the next job. He's doing well thankfully but he's a self starter and makes it happen. Has a 5 and 10 year plan and is always seeking opportunities within and outside of work to develop. I'm not ambitious really now, I've got to the level I wanted to get to before having kids, have my forever home and a snoozing baby on my lap. I'm SO lucky, I recognise that, but it was from a lot of hard work, risks, putting myself out there, research and just doing!
All the best to you, hope you do succeed how you want to!
Oh my husband was a lawyer and now we both work in the nhs and neither would recommend law or medicine as a career to our children lol.