Finding it difficult to keep up with coursework after joining a strong master's program in mathematics. How should I study?
It's all about time management my friend.
You need to learn how to adapt and the first step to start adapting is to set your timing schedule. A time schedule for the day with clear tasks, for the week..etc. You need to start working out or at the very least walk/squat for 5 minutes from time to time while you are doing a task for every 30 minutes, it will help your back a lot.
You need to use a good chair and use pillows behind your back to set your spine as vertical as possible. Since you are a mathematician and you'll need to set a lot to solve or prove you must check your pose while sitting despite having Vitamin D deficiency, sitting in the wrong pose will hurt your back, you'll lack focus and you'll have a headache.
You must treat your vitamin D deficiency by prescribed supplements. Depending on your deficiency level you might need high doses.
I hope that helps you.