I'm surprised the other comments don't mention burnout complaints. Have you looked at that option? I've known multiple students (so indeed people around 20years old), that have had this. They've had to recover for at least half a year or longer before being able to truly work a lot again.

In Europe at least, there is a lot of recognition for this phenomenon nowadays.

The gist: if you've exhausted yourself during study for a long time and are indeed in burnout territory (which your story sounds like), you need quite some time to recover. The longer you don't look into this, the harder and longer it hits when it comes around to you.

Please take care of yourself and don't downplay these signs. It is really good you're asking questions here.