Discover diary not so full? Don't repeat that mistake (5,4,6)

'don't repeat that mistake' is the definition.
(if you learn a lesson you won't repeat a mistake)

'discover diary not so full?' is the wordplay.
I cannot quite understand how this works, but
'discover' could be 'learn' (synonyms) and 'learn' is found in the answer.
'not' could be 'less' (I've seen this in other clues) and 'less' is located in the answer.
'so' is present in the answer.
This explanation may well be incorrect...