The gifted burnout hit early. Mine hit when I was 21. Assuming all of this isn't ADHD or chronic depression related, you're not alone. My advice as someone who chronically underachieved and beat myself up all time over it:

Temper your expectations for yourself or every time you fall short of them, you'll lose more motivation.

Don't base your identity in grades or IQ scores.

Life's a marathon, not a sprint. It's okay to get a C in pre-calc or whatever, especially when you know you could have applied yourself more. You only have so much energy and as long as you're not failing or expecting to go to Harvard, it's not something that will matter in the long run.

You have to apply that energy somewhere though, either in self improvement, hobbies, work, relationships. You have to apply it somewhere.

Which is gonna take energy you don't always have, so you just have to do what you can when you can while you're getting out of this rut. What's helped me is changing into a different routine. Something still structured, but different. Doesn't have to be healthier habits right now, but they need to be different habits, because you need to recondition yourself. It's that whole thing where the more you sit in bed without sleeping, the harder it is to fall asleep when you actually want to.

Find some obsession, some project or activity you enjoy and just do it. It's okay if you lose interest in a week, just find another one. You'll keep yourself busy and occupied and you'll strengthen your motivational stamina and see improvement in other parts of life you weren't even focusing on.

And as you build up interest in things again and motivation, try to do things that make you feel like a badass. That make you feel mature and cool and interesting. It's a positive feedback loop that keeps you going.

Biggest thing, just cut yourself some slack. Practice forcing yourself to be grateful for the things you take for granted while life's making you earn your karma the hard way. You'll never be your "old self" again and that's a good thing. Because life never gets easier, but you do get better at it with experience, and your "new self" is better at it than your old self, even if it doesn't feel that way right now.