no, the sums don't change i don't think
Correct.
so for the mass balance i got
[NH4] = [HCOO-] + [H2COO]
That's a little bit off - you forgot about the ammonia. You did OK for the formate and formic acid, however, at that high pH (10) you can safely assume concentration of HCOOH to be neglectable (you may use the same approach you used for [NH
4+]/[NH
3] to check if that's OK).
At this stage you should be able to write three equations in three unknown - and solve.