I would focus on the guitar, start the band, and give it my all, in stead of talking myself into something safe and sensible and selling the guitar.
I would stick with electrical engineering in college, instead of abandoning it halfway through because I thought I 'wasn't smart enough'.
I would stop worrying about trying to control how others see and think of me, instead of driving myself into anxiety, depression, and obesity through trying to convince others I was something I wasn't.
Also, I had so many ideas that I didn't get around to trying until my 30s, and it was mostly because instead of just going for it, I would share them with others first, and get talked out of it for various fear-related reasons.
I would tell more people more things about what I've already done, and fewer people fewer things that I haven't done yet but am planning, unless they themselves were good at those things or involved in my plans.
- Less relationships and more dating. Though, it might be the case that I'd want it the other way around since I did this once in my life and got tired about it after 5 weeks (traveling, having intimacy -- kissing and/or sex -- with one woman per week). I think this points serves to show that even if I could redo life, it doesn't mean it'd be better at all points.
- Not study as much and get a job ASAP. I started a first serious freelance job around 27 (being a TA before). In retrospect, I should've ditched my extracurricular study stuff and started making money at 22 and invest 50% of it. Preferably, I'd have gotten a job at 16 (20 hours per week) and started investing 50% of that. I don't regret investing late, despite the fact that I don't have the amazing compound interest of a 16 year old. But I do regret not having any focus on building up some capital. Especially because I don't have access to high paying jobs, despite being a programmer and despite delivering good work (I am terrible at job interviews as I'm simply myself and find it hard to apply to one role). I have some now and have needed to rely on it at times and it saved me a ton of stress and related mental health issues (potentially caused by stress [1]).
[1] I know because there has been one time where I did get hit by stress for a bit too long (6 months). If it would go unchecked for a couple of years, then a moderate depression might be what you're looking at.
After all these years, here I am trying to master my craft (in web development), because this thing was told to me that is the job of the future, but no one told me that it evolves so fast that whatever you learn today, in 3 months time becomes obsolete, let alone ancient!
Now, if I had to choose between technology and anything else, I would choose the latter 100 times!
It would be nice to become 16 to 20 again and give myself the chance to choose a much more traditional job, something that involves hands and creativity.
I know it's never too late, but there are not so many opportunities anymore I'm afraid...