I wouldn't say I'm extremely succesful and happy now (late thirties), and I wasn't completely lost in my 20s... the change isn't that spectacular, but the difference between me now and then is seriously big on a few levels, so here goes.

In my 20s, I was dropping out of university while the finish line was in sight, I was in a unstable and messy relationship of which I couldn't get out of, my student room was a big mess, and I had a (albeit small) debt ($15k). I had half a job in IT servicedesking which I didn't particularly like, and the relationship with my parents was deteriorating. The only thing good was my participiation in a theatre group that was quite succesful.

Although I still work at the same IT employer, I'm now a team lead and much happier with my daily tasks. I work three days a week and the rest of the week I'm working on earning money with theatre and writing, which is starting to happen (although I think I can still work a lot harder at it). I have an awesome apartment in a city centre, am no longer in bad relationships, and am happier on all levels.

The main thing that happened was that I never fully ignored the unhappy feeling I had, and that at one point I could no longer go on with (in this case) the relationship. After that, it took me quite some time to become brutally honest with myself on what I didn't like in my life, combined with the slow drive to work on my self esteem. I know myself a lot better now; I know I'm never gonna be the guy with a calm and controlled demeanor, bursting with self confidence: I have my bad days, and I'm passionate, emotional, and (as often goes along with it) sometimes slightly emotionally volatile, but I'm not supressing emotions anymore, while at the same time (and more importantly), I don't let the emotions guide my decisions anymore.

I never had a plan, I just had a nagging feeling that I wasn't happy and I should change stuff. That feeling will probably never fully go away, but I'm very glad I started listening to it and took steps to be able to make changes.