The past always seems like a better time simply because you remember the good much more vividly than the bad. That isn't to say that life wasn't actually better in the past for some (maybe you), but there is a bias just based on selective memory. When I think back to high school, my first thoughts are all good, but when I think a little harder about some of the other shit that was going on at the time, I realize that there were high points and low points just like now.
If you really think that middle school was much better than today, maybe you should try to figure out what made it better. I realize that much of the difference is situational and that you'll never have quite the same situation again (you can't just decide to live with your parents and have all expenses paid for you, for example). But what parts of your life that you can control have changed for the worse since then? How can you change these things back?
If you like the fact that life was carefree then, realize that you can still live a low stress life even as an adult. You might have to make some sacrifices to do so, but if it's worth it to you, that's what really matters. Things that are out of your control change, but a lot of things are in your control. It just comes down to figuring out what it really is that you want (what's missing now that you had back then) and then deciding how you can get it.
I have some personal experience with this. I will graduate this year and I've been applying to a lot of fast paced corporate type jobs that would leave me financially set and in great possition to "climb the ladder". A few weeks ago, I realized that I don't want that life for another few years. For now, I'm happy to make enough money to live and have time to run and enjoy life. Maybe in a few years I'll be ready to jump into the corporate world, but I'm in no rush. I completely changed the types of jobs that I'm applying to and I'm happy about it.
I guess the point of this is just to say that some choices are made for you, but you need to decide what you want and how YOU can make yourself happy. Don't worry about the things you can't change, but work with all the things that you can change.