B12 deficiency and MS can cause the same damage
True pernicious anemia is a cause of severe malabsorption of B12. When one has pernicious anemia (lack of intrinsic factor-----not actually a type of anemia), one would go down relatively fast (within many months or some years, not decades).
A person who has a less severe malabsorption can go down very slowly for decades. And that person can progress to severe malabsorption or not.
So, unless there are other circumstances (such as periodic B12 treatment) it is very likely that you do have MS.
However, even people who do actually have MS usually become low in B12. There are apparently various relationships that some researchers are still trying to sort out.
And, in fact, a percentage of those with actual MS are helped by very large doses of methylcobalamin B12 even when not B12 deficient.
So, there are reasons for you to look into B12 even if you have MS, which sounds very likely.
I will be adding much more to my B12 website, but it can help you with the basics already. Check it out.