Perhaps I didn't elucidate my point well enough. I meant to point out that I understand the phrase dum spiro, certo to plainly mean 'while I breathe, I fight' - two simple, unconditional, contemporaneous actions. This is the standard usage of dum with the indicative. If this is what the OP meant, then the phrase is just fine.
But I think that the OP's intention was to say something more like 'as long as I can breathe, I will fight' - a conditional sentence, where the outcome is dependent upon fulfillment of the condition. In this case, we need an indicative main clause along with a subjunctive proviso clause (strike my suggestion in my previous post - the future would do better here): Dum spirem, certabo. Although this might dismay the OP since the mimicry of the original quote is somewhat weaker.