Wiswas are from shaitaan. I know a lot of people,
say that but it is the truth. The same happens to me. I feel impure most of the time after ghusl but if you have let eater pass through your whole body and made sure there is no dirt on there, then you have completed your ghusl.
Note that ghusl doesn't become invalid by what invalidates wudu'.
The things which require performing ghusl
It only invalidates by what makes it necessary such as:
semen emission due to lust and desire.
intercourse (with or without semen emission)
end of menses or childbed bleeding
death (this is the only case where somebody else must ...