Welcome to NeuroTalk:

It is too bad you have to wait so long to see the proper doctor.

Food intolerances can upset people quite a bit. Fructose and gluten will cause diarrhea, in people intolerant to them.

People who use lots of antibiotics, may develop colitis or other imbalances in normal bacteria in the bowel and this leads to diarrhea.

If you use proton pump inhibitors or H2 antagonists for stomach acid, you can develop diarrhea from them. Prilosec (Nexium) or Zantac/Pepcid. This is due to poor protein digestion which feeds bacteria in the bowel, which then ferment the contents and cause gas/diarrhea. This leads to malabsorption of B12, folate, calcium, iron, zinc and magnesium, which all need acidic conditions to be absorbed.

Chronic diarrhea, causes loss of potassium and magnesium and getting low in these cause all sorts of problems. (muscle cramping is major).

Do you have this diarrhea all day long? Or only in the morning?
If only in the morning, that suggests a colon problem. I'd look at your diet carefully and see if you can find triggers.
First off stop all sugar and starchy carbs. This can starve bad organisms that are fermenting your food. Fructose in fruit, also can cause a diarrhea of its own. If you feel better on this elimination diet, that is a huge hint, that you have something that can be fixed.

The gastro doctor will probably want to test for Celiac and gluten issues, so you need to be eating wheat products before the testing, so they can be detected. So you can reintroduce them 3 weeks before the tests, so they will show up if you stop them now.

Do you know what your B12 result was in numbers? If it was very low, below 200, damage could have happened already to your spinal cord. This may or may not be correctable depending on your body's healing capability. The new cut off for normal is 400 in US units.