Must I re-observe the days of fasting concerning whose validity I had doubts due to obsessive thoughts and the inability to distinguish between semen and other fluids?

The cure for obsessive whispers (waswas) is to disregard them and pay them no mind. You are not required to repeat any of the days you have completed, as doubt about an act of worship after its completion is not to be heeded. Furthermore, the emission of semen does not invalidate the fast absolutely. If semen is emitted due to a wet dream, or by thinking, or by merely looking without repetition, the fast is not invalidated by that. These obsessive whispers have no justification. And when in doubt about what has been emitted, whether it is semen or something else, a person may choose to assign it the ruling of whichever of the two he wishes.