I don't like advice very much. In my opinion most people who ask advice really want emotional reassurance, and most people who give advice mostly want control and to bring the other person in line (and emotional reassurance). To avoid these pitfalls, I'd limit my advice to myself to stuff that was really specific and easy to execute. With that in mind, here's the advice I'd give my 15 year old self (so ten years ago):
1. Avoid [hobby you don't actually like]. I did it because my friends were doing it and because it looked good on my resume. I think there were other ways to make friends, and I don't think my resume needed the padding.
2. Spend more money. Specifically on things like clothing and belongings where I was trying to penny-pinch at all costs. Before I got a "real" job (aka a job where I could use it to pay for items instead of feeling like I had to save for my future), I would not buy clothes unless I absolutely needed them. But dressing in good looking clothing would've been worth more than it cost. Similarly for doing events like music festivals where I was always hesitant due to a financial cost that now looks like less than I would make in 1 week, but would've given me a memory/life experience that would've lasted years.
3. Pay attention to how hanging out with different people makes you feel. Specifically [one specific group of people] make you miserable every time you're around them even though you like them a lot and they like you a lot. Cut back on spending time with these people.
4. Be more ambitious when applying for college. I didn't know a) how good I looked to college's and b) how beneficial it is to go to a brand name college instead of just a "good" college. Not that I would've gotten in, necessarily, but I certainly should've tried.
5. Prioritize amazing things happening to you (travel, friendships, experiences) over being as successful as you can as fast as you can. Specifically, about half of the internships I did in college should've been travel or volunteering or "unique" experiences, instead.
6. Get a job straight out of college. It's actually much harder to get a job after you leave college cause you don't have quite as many resources (like a career center) and a lot of pipelines that are designed to scoop up new grads are no longer available to you.
In addition to that advice, I'd give a few "suggestions" for shit that I should definitely look into or consider even though it's not advice per se:
1. Travelling to low cost-of-living areas and living there for a while. You can live in Vietnam or Thailand, to name a few, for the same price you can live in America but you'll have your own apartment and eat at restaurants every meal, not to mention you'll experience different cultures and grow a lot as a person.
2. Read the book "Impro: Improvisation and the Theatre" by Keith Johnstone. For the person reading this post, I wouldn't necessarily recommend it to you (unless you're an autistic person trying to learn social skills), but for my past self, it would put into words a lot of stuff that I was trying to describe and understand for several years.