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Well, what can I say... Some of the problems you're describing are definitely not related to the breakup situation. You are in a situation where you are unhappy with your life in general and worry about your future. When I'm in a situation like this I worry deeply too, when it's stressful enough I may even get depressed and have panic attacks. Usually these feelings pass. And after a while you realize that self-doubt is your own worst enemy.
The longest day of the year...I had almost forgotten about that. Here summer has barely even begun and it's gonna be relatively sunlit til October or so. I'm a nightowl anyways, but I've never quite understood seasonal mood swings. A snowy winter morning can be quite as sunny as a summer day sometimes.
I understand that you feel dread and emptiness with what you thought was your best friend gone. But from what you wrote previously, I don't see how you could be any happier with her more prominently in your life. You'll get used to the emptiness, and as dark as that sounds, that will make you stronger. And things are going to get better. You will find other opportunities and other people. Other people help a lot too, even if it's just someone to have a decent talk with once in a while. If you can, try to find some people to casually hang around with, several people if possible, so that you don't have to rely on a single person too much.
Actually, that breakup I mentioned earlier, I had some friends around, but I spent like 6 months stone-drunk. Can't recommend that really. Also threw myself into lots of relationships that didn't work out, because if you aren't emotionally ready for something, it won't work. So maybe learn from my mistakes and don't try that.
And you know what, a girl that turned out to be less than a footnote in my life once broke up with me quoting Nietzsche's immortal worlds: "what does not kill you makes you stronger". And she was right.