Why do I reminisce so hard about the past?

You're thinking back to a time of when the possibilities in front of you seemed endless... and you actually had HOPE for the future. 

It's because you aren't happy with your present. 

This one hit right home… lmk if you find a cure

Go out and meet new people. Do new things/focus on your goals. I used to have this issue too and that fixed it. 

I feel this way sometimes. Think about old friends who have gone off in different directions, stuff we used to do, etc. like you say, I don’t even remember even being that happy or anything. It’s the power of nostalgia I suppose. I’m actually in a better spot now but I need to stop wasting time, try to fill my day with more stuff to do and more hobbies instead of the usual. I don't have a cure for it but I don’t think it’s unusual to feel the way you do.

What rank r u on csgo?

This is a first world problem. At least you don't have to worry about food or water like people living in Africa

you need to set concrete, meaningful goals (whatever that means to you). Then make a plan and execute that plan one day at a time.

The only times I used think about "the good old days" and any regrets from the past is when I am sitting around doing bumfuck nothing. 

I think it's a common theme on this site for a significant amount of people to put their entire worth as a human in their career progression. As you get older, you realize that while money is a great thing to have, there is a lot more to life. Ultimately, you're not going to be talking to your loved ones about deals you've executed on your death bed and you can't take the prestige or money with you either.

I think consistent nostalgia is a result of not being fulfilled in the present and probably means you need to work on building, repairing, or gaining new relationships in your life. While I enjoy the occasional video game, it's not at all a replacement for genuine human connection. Even if you aren't achieving something in your career, I'd recommend getting outside, joining a few clubs, intermural sports, bowling leagues, cooking classes, nonprofits, or whatever else and broaden your horizons a bit. The other purpose that serves is that you won't be idle enough to sit around and think about all of the good times... you'll be active building a new/better reality for yourself.

I always look back and think fondly of my summer going into my freshman year of college. I had zero responsibilities and expectations, and it was one of the most enjoyable periods of time in my life. All I needed was $20 to chip in toward a beer run and my friends and I partied in some capacity every night. Not sure if you have a similar time period you reference, but I think the better question is, how could you not? 

However, for context, I was a very sub bar HS student who had no future roadmap, was constantly getting in trouble, and had sky high anxiety at all times, because I knew I was on track to amount to nothing. 

For me, life got real in college, and unlike most people, my partying stopped almost entirely, and I started hitting the books. I lived in the library, had a small social circle, and rarely had GFs or even went out with girls. Ever since then, I have been grinding pretty hard, and haven't really felt "care-free" since. What my brain fails to acknowledge is that in the moment, back during that one perfect summer going into freshman year, I was actually not that happy either. I was devastated because I was going off to the other side of the country and my friend group was splitting up for the first time. I was extremely concerned that I wasn't college material due to hardly applying myself for the entirety of high school. I was bummed my friends were scoring more girls than me, seemingly most of the time. My relationship with my immediate family was always sensitive because I wasn't meeting their expectations (which in hindsight were 100% reasonable). 

To summarize, the issue is that as life demands creep in our 20s and through our 30s, it is natural to look back at a simpler time, and to romanticize it. Life is always relative, and now that things have "worked out" for me, the premise of being able to relive that summer (especially given what I know now) couldn't be more idyllic. 

To return to your thesis question, what do I do about it, I have a couple thoughts:

1. I try to appreciate the fact that I was able to make a lot of mistakes and had a few years where I just focused on having fun, disregarding the consequences at all costs. I look back and realize how fortunate I was, and that I was able to regain focus academically and career-wise, to mend those family relationships, to figure out why my friends got more girls than me, and to hold on to all of those life-long friendships, to do all of that with zero long-term consequences. That is truly privilege.

2. I also try and hold myself accountable for glamorizing that summer too much. I make a strong effort to remember that day-to-day, it really wasn't a utopian lifestyle, and that I was struggling a lot. I also try to remember just how close I was to making irreversible mistakes, and that if I hadn't turned a corner after my brutal 1st semester performance, I would likely be working a dead end job with no future. 

I hope this narrative isn't too specific to my own experience to lose the message I'm trying to develop, one that relates to a far broader audience. In short, life gets meaningfully harder, and there were certain periods of most young people's lives that were simply more care-free, and it is only natural to long for that. Unfortunately, life doesn't have a reverse button, and our increasing responsibilities will continue to grow well into our 40s and 50s, which means we can only do our best to make the best of our free time. 

One thing I constantly do is try to really ensure I enjoy my limited free time is doing an annual trip with my friends. It is one of the only times I stop being unbearably frugal and I go all out (last year we rented a $2k a night villa in Hawaii for a little over a week). I do my absolute best to save my meager PTO and spend it on a worthwhile endeavor. The older I get the only thing I continue to appreciate more and more is my time with the people I care about. No matter how brutal work is, I make a constant effort to see my closest friends or family every single weekend. Tonight will be a classic rock concert with my old man. I have found that keeping these relationships as high priority and treating them as essential (just like critical routine maintenance, e.g., going to the gym, going to the dentist) has allowed me to live in the present as much as possible.