OK, quick, find the priest who baptised you and find out if he's been living a good life or a sinful one. This investigation will tell you whether you are actually validly baptised or not.
The Donatist position is one designed, apparently, to introduce doubt into the entire sacramental priesthood.
SD
If the priest who baptized you was in some way unfaithfull (examples abound today)... I can at least see why human logic would cause someone to question the validity of the sacrament. This fails to recognize Who it is who takes the oath, but is at least understandable. Neither theology is correct, but one is more interesting than the other. I can see mis-informed cafeteria Catholics having the same problem.