First, I want to say I am sorry to hear about your cancer, hoping the best for you.
20 is not so far behind (I just turned 29), and none of my regrets are linked to finances. Well, maybe in the sense of how to pay for what I regret not doing, but it has more to do with life experiences. I wished I traveled like many of my friends did, with only their backpacks, spending months or years in other countries. I guess I just wasn't much of an adventurer (still don't feel I am), but I feel it's becoming harder to conciliate the fact I now want to be established, but at the same time, I feel like I missed out on many things I should have done BEFORE getting established. It's a mixed feeling.
I don't regret going to college and university. Getting my BA, then MA with honours, was extremely rewarding on a personal level, and extremely fulfilling intellectually.
On the other hand, I have changed over my 20s, and my interests have changed. I now have higher education degrees that will be helpful in my career, but I sometimes wonder if I should have chosen another field, one that would make me more marketable. Again, it's a struggle between ''following a passion'' and ''being strategic about my career'', and my choice then was to follow my passion. I guess I'll always wonder where I'd be today, if I had chosen another path (would I earn more, be debt-free already, own a house already, be married already, etc.)
Also, I was in a long-term relationship from 17 to 25, then my current relationship from 26 to (hopefully forever!). I never feel I missed on the sex experience. To be fair, I have never been interested in having many partners, so it's not a regret that I carry. Just like I don't regret not going out more in the bars and so on. It's never been part of my interests. In other words : do what you enjoy, try what interests you, but don't feel as if you have to absolutely do this or that because of your FOMO. You know yourself best. Do what makes you happy today and you wont regret your past.