Hey, I feel like this all the time. It's really scary when you're not used to it. And always frustrating, as you say.
I'd say it's the depression too. That's what it is for me. The thing is to realise it now and try to hang in there. As hard as that sounds. If you don't recognise what it is for a while, and just let things slide as i have, you'll get to a point where everything is suddenly near impossible and there's no visible way out.

It doesn't help school