Usually on this site and Quora, when people have a post like yours, the posters usually offer a piece of advice, that in my humble opinion, advices are useless because usually they say more about the advice-giver, what they wish they could've done when they were younger (doctors/lawyers who studied hard to make bank, advising all students to have fun when they're young but how did they get there?), or tout their own successes when the advice-receiver may or may not have the same background to be able to replicate it (people here with STEM background touting meritocracy and hard-work will eventually get everyone a job but when did you start learning coding and under what circumstances??).

As a 29-year old, I'll offer instead my own personal regret about my 20's without any panacea, I hope that it is relevant to your stated idea even though it may not seem so at first:

Last night I came home after going out with a bunch of friends from a startup at a "reunion outing" that we all used to work at several years ago,

We are all 28, 29, 30 now and we were 26, 25, 24 when we were hanging out everyday at work and after work; and past the superficial remembrances of the "all fun times we had," inside jokes of what-he-said, what-she-said, casual bantering at the pool table and the double high-fives for the ladies and low ass slaps for the bro's after the final game, on the back of the Lyft ride home, I thought about how we never ever really fought.

Not talking about general boorishness caused by alcohol and clashing sensitive male ego's, nor the passive-aggression between friends or acquaintances where perceived slights/differences built up but never confronted, beef never squashed instead squished down underneath the social surface that years pass by, your group's "happy hours" turns from a "thing" into a remembrance - that you heard only about XXX's wedding from your other friends who had been invited but you feel only slightly annoyed because XXX has already become someone who you used to know.

But really fight in a moment, air out your differences, coming into a fight, knowing that you or the other person may not come out at end as friends anymore, but you have a hope to salvage things, out of a conviction to be authentic to yourself and the other person, out of an intent to love the other person even if there is a such deep well of negative emotions, frustration, hatred, feeling of injustice and inspired self-insecurity, that you can't help but to still respect/admire the uniqueness/individuality of the person and even a wisp of self-reconsideration of your own part in the sordid affair; and hope you guys might come be able to come out the other side.

This is the my biggest regret about my 20's. That I have always ducked all my opportunities to fight.

Instead of accepting the up's and down's in any natural relationships, I took every setback, every feeling of feeling stagnant as an outlet to push people away. Underneath the thin sheath of rationalizations is a dread of knowing myself as who I truly who I am if I were to fight, I'll be exposed. So it is with this never-said but oft-acted upon notion I've come away with a decade of superficial trinkets instead of battle scars, and without the satisfaction that I've truly ever loved.