Without knowing what the problem is, it’s very difficult to know what self-help measures might be effective. Although some people swear by them, there is no compelling evidence dietary measures actually work for MS - and accordingly, no orthodox neuro will recommend them - other than a balanced, healthy diet, suitable for everyone.
And that is assuming your problem even was MS - you are very far from being able to say that. If it was one of the literally hundreds of other conditions that can produce similar symptoms, I don’t know whether diet might be helpful in some cases, or not. But again, it would depend on knowing which condition it was. Random dieting, without knowing which specific problem you’re trying to address, is unlikely to achieve much, I feel.
If your vitamin D is on the low side anyway, it certainly wouldn’t hurt to take vitamin D supplements, especially with the onset of Winter. Low vitamin D is NOT a diagnostic indicator for MS - it’s common in the general population too - nevertheless, a lot of people with MS are deficient, and there’s mounting evidence of a link. So it wouldn’t do any harm to up it a bit anyway.
I would say that’s the only self-help behaviour for which there is pretty strong evidence, now - that and quitting/reducing smoking, if you’re a smoker.
Vitamin B12 is another supplement people often take, as, like Vitamin D, many people with MS are found lacking. But, if you’ve already been tested and found fine, it would probably be a waste of time/money to supplement that.
I really do think the answer (not the final answer, but at least the next step) is to go back to the doctor and insist you’re not happy. Fiddling around with diet in the hope of finding something that helps, more-or-less by chance, is not the most you can be doing to help yourself. If you honestly can’t face returning to the same doctor, then the answer is to change doctors.
You talk about diagnosis being “a long way off”. I’m sorry, but one thing I’ll guarantee is if you don’t go back to the doctor, you will never get one. You’re not really “waiting for a diagnosis”, if you haven’t taken the first step of making clear you’re still looking for one.
Of course, if you decide to wait until you’ve changed doctors anyway, that’s entirely up to you. I wouldn’t criticise anyone for not aggressively pursuing a diagnosis, as it’s a very personal choice. With hindsight, I ignored symptoms for years, until it became impossible (you can’t really ignore waking up and finding you can’t feel your feet, can you?) But if you don’t want to keep pushing, the alternative is to accept the uncertainty of not knowing - which you don’t seem keen to do either.
There’s no “right” answer - it’s your health, your choice. But as you’re keen to do the most you can for yourself, I know which I’d choose.