Why is my signal attenuated after applying a HighPass-filter?
The correct answer to this question has been provided by: Stanley Pawlukiewicz but has since been removed. 


  Your filter attenuates the high frequencies of the total timeseries,
  signal and noise.
  
  Your results are expected.
  
  You really can’t perfectly remove just the noise.
  
  When the noise and signal overlap in time and frequency, you have to
  make a tradeoff between attenuating signal and attenuating noise, by
  adjusting the cutoff frequency.