You say that you're getting enough protein, carbs, etc, so I don't know why you would feel so run-down. It may be you're not eating enough, but eating too much is also possible, even on a vegan diet. Unless you give us more info about how much of what you're eating, we can't really say.
As others have mentioned above, there's no vitamin deficiency that could develop this quickly- so we can pretty much rule that out, at least.
I won't deny there can be stubborn psychological/habitual cravings for meat, etc, but even that shouldn't make you feel physically weak.
Peasant (1963-1972) and Fluffy (1970s?-1982- I think of you as 'Ambrose' now)- Your spirits outshone some humans I have known. Be happy forever.