I absolutely miss high school. And I've been out of high school for 4 years now. We were all SO excited to graduate and just be done with high school, but once I started college, I really missed it.
I missed the familiarity, the same old friends, the routine, my hometown, everything.
I know what you mean about the ups and downs, too. Like, when I got to college, I was nervous but excited about meeting new people. But when that wore off, and I'd talk to my friends back home, I just wanted to be there so bad. Then, I'd have a great day (or few days) with my new friends, and I didn't miss home as much.
You'll get used to it. It's a huge transition. Sure, millions of high school students make that transition every year, but that doesn't mean it's going to be easy for you. It sure wasn't easy for me.
Believe me, though. The first three months are the hardest. But when it comes time for Christmas break, you'll realize that, hey, you're going to miss your new friends and college while you're gone. You'll be excited to go home, but you'll be excited to go back after break, too.
Soon, you won't miss home nearly as much. You'll probably never stop missing it, but it'll get WAY easier to deal with.
Also, don't feel bad about looking at pictures from high school and just remembering good times. That's normal. Hell, I STILL look through my yearbooks a few times a year and I graduate high school YEARS ago. And now that I'm done with college, I have some of the same feelings about my college friends. I miss them A LOT; I miss college, but it's easier to deal with this time around simply because I know this sort of..."grieving" process is normal.
Good luck to you.