How to use fitting function to seperate different frequency signal?

I have a data,which have some high frequency noise;I want to sperate them by using fitting rather than fast fourier transformation.(Blue circles are high frequency noise)

I used 'sin1' to fit the date,and i got 0.9962 r-square for data.

I want to divide the data into two groups(Low frequency term and high frequency term) without using fft.