Furthermore, you don't need to be a carnivore to achieve ketosis.
You have more options:
- water fasting (limited amount of studies; but humans in the ancient past did practice water-only fasting) - another form: Buchinger fast, where you limit your calorie intake to 500 kcal. You can drink juices such as coconut water. There is a clinic in Germany where people pay to fast. I think this is a littlle easier for people to get in. Helps you keep your electrolytes up (salt, Mg, K, Ca).
However, I am not sure if fasting alone will help you with a mental condition. It helps your body and brain though, but how much it will alleviate your mental issues is an open question.
I am also unsure if carnivore diet will be good overall for your body. Again, it is a way to get into ketosis, but there are more ways... I would argue more for less carbohydrates and more protein (tyrosine, tryptophane), if on a mental condition. Don't forget choline (acetylcholine). I vote for the Valter Longo approach.