Understandable. I recall similar feelings at a very young age, actually when I finished pre-school and was moving on to start grade 1. I still remember the emotion. 

My best friend I met in high school. Your buddy may be moving away but the first thing to remember is that this does not mean the end of the friendship... it may even strengthen it over time as you both go through radical change and crave the known and stable.

High school will be a new chapter and you'll be busy adademically and socially so you wont have much time to think about the changes, rather live them. 

In terms of practical hints... a couple simple ones come to mind. Focus on hobbies. You're a stacker so there's a start. Fishing too for you.

Additionally exercise is a panacea and can improve the outlook of almost any situation. I've been getting up super early these days to fit in some exercise and have been feeling much better during the day. Why dont you join me on this little project... could even keep each other accountable (time zones excluded of course)!

Lastly, as you're doing now, blogging about how you feel as you transition can help formalise your thoughts in your head and process things much more cleanly. 

You're already doing that but I encourage you to hit the keyboard when you feel down or confused or even happy. The mind needs structure to process things and blogging helps that process..

Best I can help with the time I have atm budddy. You've got a bright future grom what I've read about you. Enjoy it!