I think a good way of approaching this quote is by examining its expression in modern English:
If you’re being fair, you’ll have to describe me as someone who loved too much, but who wasn’t wise about it.
The most apparent technique here becomes
high modality with the use of the word
"must" to stress the need for describing him as
"one that loved not wisely but not too well" in order to show fairness and avoid exaggeration, which is also emphasised by the preceding quote:
When you shall these unlucky deeds relate,
Speak of me as I am. Nothing extenuate,
Nor set down aught in malice.
In modern English, this expresses the following:
When you record these sad events in your letters, please describe me exactly as I am. Don't tone things down or exaggerate them out of hostility.
I hope this helps!
