Can't sleep all night until my morning alarm goes off - then im exhausted

Hi guys, pretty much what's in the title. I've had insomnia since i can remember but most of the time i can get around an hour or two of scattered sleep on a bad night. It's currently 6:45am and i didn't sleep literally at all. I couldn't even have hope of falling asleep until my alarm went off to get up. Suddenly im so tired and my body feels heavy like it doesn't want to get up.

My job doesn't suck, its not a high pressure day, everything is completely normal, i went to bed the exact same time as every other day, and yet my stupid useless brain decides we aren't going to sleep until literally the only time we can't sleep. I take magnesium for sleep which has worked great every other time except now for seemingly no reason whatsoever. Anyone dealt with this? Not the first time it happened.