What is the treatment for vitamin B12 deficiency?

Hello doctor,

I am an 18-year-old and take no medications. I have been struggling with a range of symptoms for over three years. My first symptoms were palpitations and tinnitus. Palpitations originally occurred once a day but now happen possibly hundreds of times every day. They tend to happen when my heart rate is decreasing. Tinnitus has remained consistent at about one episode per day typically lasting 10 to 20 seconds in either ear. I have depersonalization or derealisation and visual snow which developed slightly later. I have a frequent dull frontal head pressure and headache but it is not painful. I am finding it difficult to concentrate and have had severe brain fog for two years. I also have a loss of smell which is intermittent. I have foamy urine which has developed in the last year. Less foam tends to correlate with feeling slightly better generally. I feel I also urinate more frequently than I used to. I have begun twitching in the last year and it has been increasing in frequency to multiple episodes a day. It can be anywhere on the body including the face, legs, arms, hands, feet, and stomach, and will last for about 10 seconds. I also occasionally have episodes of painful general headache, become very shaky and cold, and also become very dizzy and nauseous and these typically last an hour. During these episodes, my other symptoms usually decrease in severity.

I have got blood tests for vitamins, and minerals along with glucose and thyroid. I have had glandular fever that I contracted from a friend two years back. I have had EKGs and an echocardiogram both showing no abnormalities. I had covid last year but most symptoms presented prior. I have done an at-home urine dipstick test for protein, ketone, and others but have not found any issues. I may also have weight loss as a symptom but I am not sure. My most recent blood test has shown high alkaline phosphate. I am concerned about all the symptoms that can related to a serious disease. Kindly suggest the diagnosis.

Thank you.