*Always get an Active B12 blood test if you can. It tells you how much is being actively used by the body. Normal B12 just gives you total B12 which might be just sitting around doing nothing. You can (as in my case) show adequate B12 on a normal test then find you are scraping at the bottom range for active.
With regard to Vit D you need to aim for being high in the range even the upper range is low in the UK. Dr was fine with it being 40! Despite cramps and hypothyroidism where you critically need vit D
Folate is an absolute MUST as pp said.
Magnesium and Selenium are also key. Virtually no women get enough magnesium. If your adrenals are shot to pieces, from sustaining high levels of cortisol, you need plenty magnesium and lots of vit C, and B complex minimum.
Multivitamins don't cut it. You need quality forms of the individual vits in fairly high doses.*
I second ALL of this with bells on
But will add. If your B12 results are in range & GP not listening/understanding your symptoms. Look up "functional B12 deficiency" on the NHS page & shove it under their nose
My DD has this diagnosis as her blood results were in range & it took a fight to get her injections, but boy was it worth it as after 2 years of severe ill health & no help at all, (even hints at munchausens by one shit doctor who resented my getting second opinions
) those 6 x B12 loading dose injections had her out of a wheelchair & sleeping almost normally f more, in under 2 weeks