Brain fog or dementia due to b12. Hope.

I am suffering from insomnia from 10 years sometimes I sleeps good and sometimes not. Few days before i was diagnosed with high cholesterol and triglycerides level. Doctors says me to lose weight and eat healthy. In the process of losing weight one night I couldn't sleep in morning I feel very tired and brain fog. I went to hospital they diagnosed me with Deepression and put me on high antidepressants. One months I was going through the hell and no improvement at all. I feel like brain fog and dementia constantly and couldn't sleep. I lost all my hope in life. Finally I test b12 level which results 75pgml. Doctor put me on b12 injections alternate days. But my brain fog or dementia is constant l. In evening it worsens. Can I be cured because I have no hope.