T-SQL, 172168 bytes
SELECT STRING_AGG(c,'')WITHIN GROUP(ORDER BY
x)FROM(SELECT substring(@,number,1)x,sum(1)c
FROM spt_values WHERE type='P'GROUP BY(SELECT substring(@,number,1)x FROM spt_values WHERE'P'=type)x
GROUP BY x)x WHERE x like'[a-z]'
This will work for input fewer than 2048 characters. The output is ordered according to the position of the letter in the alphabet