[Borek]

For A and B, the calculations were done at pH 0, but, for C and D, calculations were done at pH 12.5 for final.

If so, perhaps these numbers are correct (at least they don't look illogical). But you have to be a bit more specific in future, as this

I have solved for A, B, C, and D in terms of moles after finding the total number of moles of histidine.

A = 0.001231 mol
B = 0.000019 mol
C = 5.84*10^-7 mol
D = 0.001249 mol

is not clear.