So I don't really know how to ask this question because I can't quite explain what I really mean or want to ask.
I am now 30 years old and I couldn't be in a better situation than currently. My job is fun and providing me enough money to live a happy life and pay my own built house (I am a nurse). I have way more free time than lets say even in my childhood. I remember coming home from school and feeling anxiety cause of exams. I remember nursing school, it was hell for me.
Now all I have to do is go to work and when my shift is done I am off and can do whatever I want whenever I want. I don't have to ask parents to drive me somewhere, I have my own money, I have my own house, I can play video games all day... and still, I don't know why but nostalgia is real.
I am not even sure if my feelings are real. I can only give silly examples like coming home from school, logging into world of warcraft with friends and having a blast. But I don't think it was like that. We were all just in skype and everyone was minding their own business in that game. We had good laughs though.
I don't know. My life should be so much better than it was 15 years ago but I miss the 2000s era. It all just felt so different. I remember the hot summers going to the lake with friends.
Now? I don't know. Maybe it is because it seems like I am the only one that has so much "time" and no one really joins anymore. While I have a wife and no kids and most of my friends don't even have a wife yet (so no kids....) they still are somehow busy and don't go to the lake anymore, don't play video games anymore, don't do anything. I actually wonder what they do all day.
Life felt more exciting back then. Maybe because I had goals and now I am "done"? But this should feel good. I am happy that I am "done". House, wife, job. Those were the things I wanted back then and now I am more than happy to have those things.
Imo many of the comments here are missing the point, and it sounds like you may not be familiar with the breadth of other life experiences.
That said, I think one of the key things you've described is experiences vs things, and time vs loneliness. Having arrived doesn't make people happy. Having fewer problems doesn't (necessarily) make people happy. Living in the moment, finding connections, building new experiences, finding ways to help people can all help.
That said, you're probably also experiencing some existential questions more. The future branches of your life are no longer the focus. Mindfulness can help with that. When life has changed, you don't feel fulfilled, and you have everything that society says you should have, it's still easy to wake up one day and realize you're depressed. It sounds like you're starting to look for answers. You may be surprised to find that there really aren't any. And that's ok.