You will want to find out the numbers of your B12 test. In the US "normal" is typically very low. If you are below 400-500 you should be taking a supplement every day.

Vit D also impacts depression. There are studies out showing improvements with depression as the blood levels normalize.

I take 4000IU D3 myself.

Here is a link to the newest information, put together at Univ. South California, San Diego Medical school:

http://neurotalk.psychcentral.com/thread92116.html
It is long but it explains alot.

Once your depression lifts, you will feel less pain. The two are connected.
Other things that improve depression are folate, and DHA (this is a long chain fatty acid in fish oil).

What was the drug your doctor gave you? Elavil (amitriptyline)?
This drug has anticholinergic effects...not good for MG patients.

Shari--- are you using any osteoporosis medications like Boniva?
Those drugs can cause muscle pain as a side effect.