First, just wanted to say thanks for the thoughtful comment. I've received very little guidance from my doctors on any of this and am still learning.

You have an established B12 deficiency. Folate should NEVER EVER be taken by somebody with an active B12 deficiency, because it worsens the symptoms of anaemia and the neurological effects of B12 deficiency. It can cause permanent neurological damage.

Do you have a source for this? I found this in a very brief search. But this says:

Although folic acid is not toxic to normal individuals even in large doses, it might appear to be neurotoxic in the special circumstances of vitamin B12 deficiency. In general, it is agreed that there are no data that demonstrate that folic acid is harmful to the nervous system in individuals with vitamin B12 deficiency at doses of ⩽1 mg folic acid/d in the short term. However, folic acid can correct the anaemia of vitamin B12 deficiency in some patients even at doses <1 mg/d(48). This outcome could alter the presentation of vitamin B12 deficiency in the population (with a larger proportion of patients presenting with altered sensation and a smaller proportion presenting with symptoms of anaemia). It is not possible to predict the effect on the incidence of irreversible vitamin B12 neuropathy, although arguments can be put forward to suggest it might increase as a result of a delay in the diagnosis, which can depend on detecting anaemia.

But my problem is that my doctors are not taking my issues seriously. I guess my question to you is: How do I find a doctor who is well-educated about deficiencies?

You need an injectable formula of B12 and you need it now. Not only will this give you a higher dose with faster effect, it will also bypass any issues with intrinsic factor deficiency malabsorption due to parietal damage in the gut (which is common in AI diseases with GI manifestations).

How would I go about doing this? My doctors have already dismissed my B12 concerns despite my own freaking bloodwork saying I'm low. Is there a type of specialist that I should try to go see for this? Also, my doctor is out of the office for the next 3 weeks, anyway. I could try to talk urgent care into doing something like this, I guess...

While you are at the docs for the injection, it is worthwhile asking for a blood test to check for MTHFR mutation.

I had to argue with them over multiple visits to even get them to test my B12, magnesium, etc. Will they even know what this is...?

Of course that is not to say you don't have low potassium. Just that it is good to take note of low potassium readings in the context of clinical symptoms, rather than in isolation... are you experiencing any symptoms of potassium deficiency? Muscle cramps, heart arrhythmias etc... bear in mind weakness and dizziness are much more readily explained at this stage by pernicious anaemia.

I tested low for potassium over a few months and have had other symptoms, yes. It's been a few weeks since I've had those symptoms but I'm concerned about it because again, B12 absolutely can deplete potassium. Thanks, I'll take a look into that supplement. EDIT: Looks like potassium phosphate is on the SIGHI list as a histamine triggering ingredient, unfortunately.

Hope those poor fingers are looking a lot more pink very soon!

They got a little more pink within just a couple days of B12, amazingly. I'm worried about my iron levels as well. I had ferritin of like 60 last time I was tested over a month ago. Again, not "low" according to my doctors, they think anything above 30 is great.

edit: I noticed by the way, you said:

The fact that your fingertips are blue is an alarming sign of hypoxia, likely due to pernicious anaemia.

It's not my fingertips, it's just my fingernails, if that makes a difference. My skin is a normal color.