Fill NaN values from previous column with data

I have a dataframe in pandas, and I am trying to take data from the same row and different columns and fill NaN values in my data. How would I do this in pandas?

For example,

      1     2   3     4     5   6   7  8  9  10  11    12    13  14    15    1683  27.0  29.0 NaN  29.0  30.0 NaN NaN  15.0 16.0  17.0 NaN  28.0  30.0 NaN  28.0  18.0

The goal is for the data to look like this:

      1     2   3     4     5   6   7  ...    10  11    12    13  14    15    1683  NaN  NaN NaN  27.0  29.0 29.0 30.0  ...  15.0 16.0  17.0  28.0 30.0  28.0  18.0

The goal is to be able to take the mean of the last five columns that have data. If there are not >= 5 data-filled cells, then take the average of however many cells there are.